Exactly what it says on the tin. Feel free to list however many you want, mine are:
The dragonfire adept handbook by janusjones: this is probably my favorite handbook. The handbook showed me that what i originally thought was a warlock with fire breath, was actually a very interesting and versatile class. It is extremely comprehensive and full of personality and humor. You can find it at the WOTC forums. Unfortunately no one has transfered it yet. I may have to take up the task despite my technical ineptitude. Here's an excerpt:
"Gestalt: Hooooo boy. Gestalt makes for some crazy, kung-fu, epic, super-powered, world-shattering, DBZ-style action. Dragonfire Adepts can be an AWESOME half of a Gestalt combo, and make you look and feel like sex on the beach - dirty, naughty, and stylish. First choice here would be a caster, preferably one with access to metabreath spells a la the Draconomicon. These include such hits as Rebuking, Dispelling, Stunning, and even Enervating (negative levels on a breath weapon - yes PLEASE!). Other good choices could include skill monkey/casters (yay for Beguilers!), or even a nice fat Aoo-based warrior-type (thin Crusader with Stand Still - you'll be your own backup). If you're going the warrior route, be sure to trick out the metabreath feats, running up your recharge rate, and then use the other rounds to whomp your foes with martial strikes. Good times. Hard to blow a gestalt, but focus on complementing the DFA's significant at-will de-buff and utility abilities with damage and casting and you'll kick butt and take names. "
See what i mean by "full of personality and humor? Find it
here I'm feeling really tired right now. I'll edit in some of my other favorites later.
Edit: another one i really like is the new dread necromancer handbook. For some reason it is one of my favorites despite having a few flaws. I like the tone and it was the first handbook I ever saw that had a section on roleplaying. It being one of the first handbooks i had read probably had something to do with my liking it too. Find it
here