[Elfin]

Multiclassing: Half a Sword
Thanks to Essence_of_War, ShneekeytheLost, and Draz74

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Barbarian

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Unfortunately, it doesn't give you much you don't have already. Full BAB? Got it. D12 HD? Got that too. Hulk smash? No problem for ya. DR? You have Stone Dragon for that. Rage? Maneuvers are a much more effective way of increasing damage output.

In fact, there's only one real thing you can get from it. A single-level dip with the Spirit Lion Totem from Complete Champion lets you trade out the 10' speed bonus for Pounce. That means you don't need to wait to get Pouncing Strike, and can use it more frequently. Of course, since you've already got it, you may as well pick up Intimidating Rage and the skill trick Never Outnumbered for some battlefield control to go with your pounce. Other than that? Give this a pass.

Draz74 has a few additional points:

Eh, Whirling Frenzy is still pretty nice. Not worth it by itself, still, no. But if you're dipping for Pounce, I'd suggest taking Whirling Frenzy instead of Rage + Intimidating Rage.

A second Barbarian level is also pretty viable, if you use some other ACFs, since it doesn't lose you any Initiator levels. In particular, the Wolf Totem gives you Improved Trip instead of a redundant Uncanny Dodge. Of course, you have to ask your DM if he'll allow you to take Lion Spirit Totem from CChamp and Wolf Totem from UA.

Bard

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Bards and Warblades go together like healbots and beatsticks. There's a feat that even makes this easier: Song of the White Raven. It lets your Bard levels and your Warblade levels stack for determining your level of Inspire Courage. There's been a lot of thought on how to maximize Inspire Courage, I suggest you look into it. While you are at it, look up Dragonfire Inspiration.

What you lose: Two initiator levels, 1 BAB, 2 maneuvers known, 1 maneuver readied, and Stance Mastery with a traditional 4Bard/16Warblade build

What you gain: Inspire Courage to give Morale bonuses to everyone's attack and damage, and because it is a Morale bonus, it should stack with just about everything else (other than another morale bonus, of course). Possible Dragonfire Inspiration to give a handful of d6's to everyone's damage rolls. Mind you, this includes yourself. Suddenly, you will find yourself being able to hit a lot more often, and doing a lot more damage per hit as well. And so does your allies. With Lingering Song, you can get both of them going simultaneously long enough to end the fight.

If you are considering being the 'party leader' who wants to help and support the party as a whole in a meaningful way, and already considering White Raven to do so, then this is definitely a very viable option for you.

Cleric

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Cleric dips can be invaluable - and with Complete Champion's line of Devotion feats, a warblade can extract a noticeable benefit from cleric multiclassing.

The Dip:

For a primary warblade, we're here for one reason: the ability to Turn Undead and thus fuel Devotion feats. Anything more than a one- or two-level dip provides sharply diminishing returns. I like dipping Fighter 1/Cleric 1 to get the additional IL, a bonus feat, and Turn Undead in a simple package.

Domains:

Try to pick ones that give you an ability that isn't directed at full casting or keyed off of your cleric level. In core, Luck, Animal, and Travel are all fine. War is good if you're not planning to take a level in fighter.

Devotion feats:

This is the good stuff, and the reason you're dipping cleric. Devotion feats allow you to burn uses of Turn Undead to get extra uses of the feat's abilities. Your Charisma score is the chief factor in how many uses of Turn Undead you can use; you should thus choose your Devotion feat(s) and your Charisma stat with your methods in mind.

Crusader

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If you're an AoO build, Crusader 2 can get Thicket of Blades. This alone is worth writing home about.

The crusader also earns 5 maneuvers known at 1st level. If you want to get some mid-level maneuvers from Devoted Spirit, White Raven, or Stone Dragon, and you've focused yourself into other areas, this can be a good choice. WRT is only 3rd level, and only requires a 1 maneuver pre-req, and the Stone Dragon school has a smattering of maneuvers that have no pre-reqs. If you're an elf who wants to become an Eternal Blade, but is mostly a Warblade, the crusader dip can help you pick up some Devoted Spirit pre-reqs.

Factotum

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When multiclassing into factotum, a primary warblade has two obvious options available: a two-level dip or a more committed four-level one. Generally, factotum dips are best for warblades who already have a high Int, or for higher ECL characters who have ways of boosting their Int through items. Since you may already want to be boosting Int to increase the benefit gained from your "Battle" line of class features, this can be very profitable.

Two-level dip

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What you gain:

- An Improved Reflex save.
- A cantrip (not big).
- 3 Inspiration Points per encounter.
- Cunning Insight - For 1 IP and no action, add your Int mod to any attack roll, save, or damage roll as a competence bonus.
- Cunning Knowledge - For 1 IP, get a +2 to any skill check.
- Trapfinding.
- A ton of extra skill points to spend on ANY(!!!) skill. Nice for helping to fill out party roles; UMD is especially valuable.

What you lose:

- 1 point of BAB.
- A few HP (d8 vs. d12).
- 1 IL.
- You'll probably miss 1 maneuver known and 1 maneuver retrain.
- You may also lose one or more warblade class abilities, depending on the level at which you take the dip. The viability of this depends heavily on your warblade's build.

Good level games for this dip:

ECL 3 - Lose out on Battle Ardor and Uncanny Dodge, but the factotum benefits should make you forget those.
ECL 5 - Factotum 2/warblade 3 works really well! You'll miss only the bonus feat.
ECL 9 - Miss a bonus feat, but keep Improved Uncanny Dodge and Battle Cunning.

Generally, this dip does best at low- and mid-levels. At higher ECLs you need to weigh any benefits of this dip against PrC benefits you may be missing out on.

Four-level dip:

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What you gain:

- An improved Reflex save.
- 3 Inspiration Points per encounter.
- One or two first-level spells (very minor).
- Brains over Brawn - Add your Int modifier to all Str and Dex checks and any skill check keyed off of those skills. This could be extremely useful to a tripping or bull-rushing warblade; synergy with the Knockdown/Knockback feats is delightful.
- Cunning Strike - Gives minor sneak attack. By RAW, you can get 3d6 out of this once per encounter on a single attack. Fairly small.
- Cunning Defense - 1 IP grants you your Int mod as a dodge bonus to AC against an opponent until the beginning of your next turn. Once per turn per foe. Useful for boss fights, or against ranged touch attacks if you don't have Wall of Blades prep'd.
- Cunning Insight - For 1 IP and no action, add your Int mod to any attack roll, save, or damage roll as a competence bonus.
- Cunning Knowledge - For 1 IP, get +4 to any skill check.
- Trapfinding.
- A metric ton of extra skill points to spend on ANY(!!!) skill. Nice for helping to fill out party roles; UMD is especially valuable.

What you lose:
- 1 point of BAB.
- 2 IL.
- A handful of HP (d8 vs. d12).
- A point of Fort save.
- 2 maneuvers known, and 2 maneuver retrains (depending on what your warblade level is).
- Possibly a maneuver readied.
- Possibly a stance.
- You may also lose one or more warblade class abilities, depending on the level at which you take the dip. The viability of this depends heavily on your warblade's build.

Good level games for this dip:
ECL 5 - This is a fun one. You come in with a ton of skills and can go straight for the 2nd level maneuvers with your first warblade level.

Weigh other levels carefully against PrC entrance and long term benefit.

Gestalt

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In a Gestalt game, a warblade//factotum can be a very powerful primary melee character. Keep an eye on the following levels:

Level 8 - You get Cunning Surge. This is fantastic, breaking the warblade's action economy wide open. You can initiate a maneuver and make a full attack, initiate two maneuvers, or do a maneuver and recovery! Talk with your DM about whether or not this is usable multiple times a round. Avoid flying DMGs. If you have an ECL 8+ Gestalt game, and you'd like to play a Warblade, strongly consider the factotum for nothing other than this ability.

If this is usable multiple times per round, you can take Font of Inspiration once (see below) to open every combat with WRT, Surge to recover maneuvers, Surge out WRT. Everyone in your party will thank you. Your DM will likely not. Even if you don't take Font, you can still use WRT, then on your next turn, recover maneuvers and surge out WRT!

Level 11 - Cunning Breach. Allows you to bypass SR/DR for 2 IP. This can be very broadly useful with your arcane dilettante ability (or use it to punch the hell out of golems).

Level 16 - Imrovedp Cunning Defense permanently gives you your Int mod as a dodge bonus to AC, though you have to be in light armor. That's a major downside if you aren't in a mithral breastplate or the equivalent. This can be much better if you're going for a more dextrous build to take advantage of TWFing.

Feats

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Font of Inspiration - If it is allowed can be very tempting. Usually the warblade has better things to do with its feats, but you could make an exception based on a couple of possibilities. The only time I would feel strongly about this is at factotum 8.

If you're in a Gestalt game and trying to nab Cunning Surge, you'll have 5 IP at level 8. Cunning Surge costs 3 IP to use. Taking Font of Inspiration once gives you 6 IP total and 2 uses of this ability. As you don't normally get your 6th IP until Factotum 11, this can be very profitable.

Fighter

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This is a great one, because depending on your build, fighter 1, 2, 4 or 6 can all appropriate jump-offs.

Fighter 1 gets you heavy armor, as well as all shields, and all of the usual ranged weapons. This is certainly worth considering. Fighter 2 and 6 have dungeoncrasher ACFs which combine VERY well with any large or powerful build race who is planning on doing some bullrushing.

I find myself proposing fighter 2 to most people, but the fact that you can get value out of fighter 1 + odd number of another useful class to maximize IL is a handy trick.

Monk

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A two level monk dip gets you 3 bonus feats, and with the UA variants, two of these could be just about any applicable feats to your build. I'm partial to the "overwhelming attack" line, particularly when I'm building a warblade who has powerful build or large size, but just about any warblade could make good use of Power Attack and Improved Bull Rush.

Notably, this also nabs you evasion, and if you're planning on using a mithril breastplate or lighter, this combines saucily with your 'action before thought' diamond mind maneuver.

The bonuses to base saves are also nothing to sneeze at.

It's also possible to get value out of monk 1 or monk 6, but I find neither of these to be as compelling as monk 2.

Paladin/Hexblade

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Almost certainly Pal/Hex 2, but potentially Pal/Hex 4 or Hex 5.

Hexblade 3 is notably a fast way to get "mettle", an ability that could be very nice combined with your diamond mind save-replacements.

Both of the 2 level dips gets you cha to saves (situational for hexblade), both of the 4 level dips get you casting, and the pal 4 gets you turning. If your build needs any of them, it's good to know you can get them with full BAB and goodies to reward you if you have a high Cha. But since none of the warblade's class features are keyed off of charisma, this is worth mentioning if it's relevant, but not really worth writing home about to everyone.

Ranger

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Ranger 2 gets free TWF. This is useful if you want to TWF, and you should know that you can probably argue THF+Armor Spikes works with this if you want a way to get another attack.

Samurai (OA)

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Like the fighter, this guy has jump out points that are both even and odd. Samurai 1 gets Ancestral Daisho, easily worth a feat (Ancestral Relic from BoED), and Samurai 2 gets a bonus feat. It also offers two good saves. Not as flexible as the fighter, but not having to rely on magic item marts to boost your primary weapon is SAUCY.

Prestige Classes: Perfecting the Sword

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Exotic Weapon Master

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Good for no more than a one-level dip, only if you're using a one-handed exotic weapon in two hands. If you are, it's worth taking.

Just make sure you're OK with taking either Exotic Weapon Proficiency or playing a race, like a dwarf, who gets weapon familiarity with the waraxe.

Revenant Blade

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Want to be a TWF warblade? Strongly consider this option, and take all five levels.

It's feat intensive, but the capstone is WAY worth it, and you end up netting more feats out than you take in.

I'll cross reference you to the appropriate handbook for more details.