The New Warlock Handbook [3.5, WIP]

[ThiagoMartell]

Dip Classes for the Warlock
Since it's similar in many ways to spellcasting classes, warlock should not give up caster levels if at all possible. Even then, some dips are very tasty and should be considered.
Dipping is usually more acceptable for melee warlocks, since they care more about damage potential so delaying invocations (or Imbue Item) a bit is not that big of a deal, specially if you consider the ruling Rich Baker has on Practiced Spellcaster advancing eldritch blast damage. For more generalist warlocks, dips should be considered very very carefully.
Binder (Tome of Magic): Binder is a very versatile class and very multiclass friendly. A single level dip allows you to get Naberius, which helps your face skills and makes Hellfire Warlock much less dangerous. Plenty of other vestiges are good for warlocks, including Paimon and Andromalius. This class has plenty of similatiries with Warlock, so much that it became part of the class beyond 3rd edition. Very good.
Cleric: Everyone knows Cleric is such a strong class and how strong a single level Cleric dip can be. If you go for Cloistered Cleric (a variant from Unearthed Arcana), it gets kind of ridiculous, even. It opens up Eldritch Disciple, one of the most potent classes available to Warlocks. When it comes to RAW power, it's hard to compete with a Cleric dip. Armor proficiency allows you to qualify for Battle Caster, turn undead opens up Divine Might, you can exchange your domains for devotion feats (including Knowledge Devotion, one of the rare bonuses to damage you can apply to eldritch blast).
Wizard: Wizards are almost as good a dip classes as Clerics. With a single level, you can get a familiar and a bonus feat (either Scribe Scroll or a Fighter bonus feat) or you can give up your familiar for Immediate Magic from PH2 (Abrupt Jaunt is so good it's arguably broken).
Sorcerer:
Fighter:
Ranger:
Barbarian:
Paladin: A 2 level dip for Divine Grace is good on Charisma heavy build, but Charisma heavy builds are not that good. I could see this being somewhat usable in a clawlock build using From Smite to Song to get Inspire Courage and going crazy from there, but it's very convoluted.
Warblade: The best chassis around and you can get Sudden Leap out of it, helping immensely with the mobility problem of melee warlocks. Good maneuvers all around, good skill list and a worthwhile dip even if you lack high Int for the class features.
Crusader: A good class on it's own, but it competes with Warblade in the ToB front and it's simply not as good as it's cousin for warlocks, even though it has a bit of Charisma focus.
Swordsage: 3/4 BAB and a focus on Wisdom (one of the least important abilities for warlock) makes this the worst out of the 3 ToB classes for Warlock. Note that the Shadow Hand feat does not apply to claws and that you can get all of the utility from maneuvers from invocations.
Incarnate:
Totemist:
Soulborn:
Hexblade:
Swashbuckler:

Prestige Classes for Warlocks:
Eldritch Disciple (Complete Mage):
Eldritch Theurge (Complete Mage):
Enlightened Spirit (Complete Mage): This class does not advance your warlock abilities. Instead, it gives some celestial-flavored abilities, +5d6 eldritch blast and 5 set invocations. Usually, this class sucks. However, in a gestalt or epic build, Enlightened Spirit shines. Since it is adding instead of advancing, in a gestalt build it basically doubles your eldritch blast progression throughout those 10 levels. 5 extra invocations is nothing to sneeze it - the problem is that they are set. However, they are still invocations and as such can be swapped away when you get a new tier of invocations. Having Planar Affinity helps, since you can swap two at a time - two when you get Greater, two when you get Dark - you only need to keep one, and it turns out one of the invocations Enlightened Spirit grants you is pretty good (Celestial Flight). In epic levels it works very similarly, except that you need to time your levels carefully.
Demonbinder (Drow of the Underdark):
Hellfire Warlock (Fiendish Codex II): Probably everyone that wants to play a Warlock already knows about the Hellfire Warlock, THE Warlock prestige class. Don't have Fiendish Codex II? No worries - it's free. It requires a sucky invocation to get in, but it's worth it. It progresses your invocations, add some damage in the form of hellfire, allows you to apply some metamagic to items (and this is a really good and often overlooked part of the class) and you even get an immediate action ability, one of the very few a Warlock gets.
The problem is that using hellfire deals Con damage and you can't be immune to it. There are three common ways to deal with it:
1) Use wands and rods of bodily restoration to keep going. The most simple solution, one suggested in the class description itself. Better if you have a familiar.
2) Dip Binder (Tome of Magic) and bind Naberius. You now heal 1 point of ability damage per round. Binder and Warlock fit like a glove onto each other and this fits both the RAI and RAW of Hellfire Warlock. The only problem is setting back one step of your progression.
3) Take Shape Soulmeld (Strongheart Vest) (from Magic of Incarnum) as a feat. This reduces any ability damage you take by 1. This is definitely not RAI and the debate on whether is not it is RAW (because what the hell does 'somehow immune' means?) has been raging over years and has never been completely settled. Check if your DM. If you can take it, hey, free power boost. Update 14/12/12: The latest 3.5 FAQ addresses Strongheart Vest.

Would the strongheart vest soulmeld (MoI 89) protect you from the ability damage of the hellfire warlock’s hellfire blast ability (FCII 90)?

The strongheart vest soulmeld reduces the amount of ability damage you receive from an attack; however, it does not keep you safe from the costs of hellfire blast because the ability damage you are taking is not from someone attacking you.

OK, so Hellfire Warlock is the bee's knees. The problem is that it's only three levels long. If only we could advance the class progression beyond that... and hey, looks like we can. Some people debate if you can actually do this, but I have yet to see any RAW argument that holds against it. I don't think that's RAI, but this depends mostly on your group. Check with yoru DM before doing it, basically. There are three ways to do it:
1) Bloodlines (from Unearthed Arcana) advance level-related class features. This would advance hellfire and hellfire only. You get your bonuses to damage, but no free invocations, no BAB, no skill points. I'm not a fan of bloodlines myself and I have yet to see a game that is enriched by adding them. Personally, I'd avoid it like the plague.
2) Uncanny Trickster (from Complete Scoundrel) is another 3 level prestige class. Two of it's levels progress class features for another class. The fluff even kind of fits - you're squeezing just a little tad of extra power from your pact with the lower realms by trickery. Might be hard to get enough skill points for all those skill tricks while you qualify for Hellfire Warlock, but if you can do it, you probably should.
3) Legacy Champion (from Weapons of Legacy) is a bit like Uncanny Trickster in that it progresses class features from another class. However, it does so on 8/10 progression. That's a potential +16d6 bonus for your hellfire damage. There are several problems with that, though. First of all, you can't take all levels while qualifying for Hellfire Warlock pre-epic. Also, you need to find/found a legacy item, and that's a whole book full of badly organized mechanics your DM might not want to read. Also, if you go for the most Legacy heavy build possible (Warlock 9/Hellfire Warlock 3/Legacy Champion 8) you're losing out on Imbue Item, the single best class feature Warlocks get. If you keep Imbue Item, you end up with Warlock 12/Hellfire Warlock 3/Legacy Champion 5. That's a slight advantage over Uncanny Trickster with a lot more to learn. Still, you suffer penalties just from using weapons of legacy and that sucks. On the plus side, if your DM allows you to pick and choose abilities for your item, it can get quite decent. My personal advice? Just use Uncanny Trickster instead.
Chameleon (Races of Destiny): Chameleon is on of the most versatile classes is all of D&D 3.5 and Warlock really needs the flexibility. The requirements lock you into a few builds - it requires being a human, changeling or doppelganger and taking the Able Learner feat. 8 ranks in Disguise can be a pain to get, forcing you to either be a Silverbrow Human, take the City Slicker feat (from the same book as Chameleon) or dip some other class. However, if you do qualify, you really should take it as fast as possible. A two level dip offers you the option you using several abilities (from a small bonus to melee to 2nd level arcane or divine spells) and a floating feat - that is, a feat you can select every day. This is awesome for the Warlock for two big reasons - Extra Invocation and item creation feats. Between this and Imbue Item, you can do any magical item.
You can go further than 2nd level, but it starts becoming more about Chameleon and less about Warlock very fast.
Cyran Avenger (Five Nations):
Ruathar (Races of the Wild):
Thayan Gladiator (Champions of Ruin):
Warshaper (Complete Warrior):