Subsystems and Expansions
Sometimes a homebrewer creates a great addition to an existing subsystem, sometimes one creates an entirely new subsystem and sometimes there just are those classes that gain a following and a boatload of PrCs and other connected material. That is what this post is all about.
There haven't been any Binding base classes other than the Dabblemaster (and now the Blasphemous Preacher) as far as I've found, but there have been a lot of additional materials created for the Binder's use, especially vestiges. A compilation of that material can be found here.
Bloodlines are something that was a unique thing added in Unearthed Arcana and expanded upon by Welknair. His Blood King class utilizes them heavily, and a list of them can be found here as soon as they've been collected.
Gramarie is a homebrew subsytem created by Kellus. A lot of extra material has been created, a collection of which can be found here.
Some people have created a lot of new invocations or other material to go with classes similar to the Warlock and Dragonfire Adept, such as ErrantX's Ebon Initiate and Channeler. Some examples of sets of invocations include the following: Animal, Celestial, Elemental, Necromantic, various. There are also ACFs.
The Limit Warriors (also including the Limit Dragoon) are a group of eight classes that all use a unique subsystem that allows them to fill up their 'Limit Gauge' as they take damage and use the points in this Limit Gauge to fuel special abilities. The Limit Knight and Limit Sniper can be made more useable by tweaking their weapons to be easier and cheaper to fix and upgrade and allowing the Limit Sniper to shoot more often. Otherwise, the classes are fully useable, as long as you keep up their Constitution score for their hit points. The Limit Break mechanic was given a new spin by Ziegander here.
Tome of Battle
Tome of Battle is probably the book with the most homebrew support ever. A lot of homebrewers have at least one piece of homebrew to their name that somehow involves Tome of Battle. Because quite a few of the classes on the Tier lists above and below are martial adepts, here is a listing of a number of homebrew martial disciplines that have the Bark Seal of Approval.