http://www.wizards.com/default.asp?x=dnd/lists/prc
The official PRC list from Wizards, for those who may need to look a name up.
I'll stick with mostly stuff that isn't from a campaign setting.
Some of my favorites...
Alienist- Yes, I am a Lovecraft fan. But these guys are just creepy and well built.
Black flame zealot- Okay, so I love cultists. I admit that. But this is another PRC with great flavor and a nice suite of powers. And you can slap a generic flame worshiping group in any setting.
Blackguard- Evil champions.... that are better built than Paladins..... and have a cool name.
Bloodhound- Really neat class. Good for PCs and NPCs. It really delivers what it sets out to do.
Divine disciple- A Forgotten Realms class that works well for any divine character. I kinda wish the higher levels of the base class cleric looked more like this.
Divine seeker- A secret agent for the church. Makes a real neat take on the religion oriented character.
Dungeon delver- The perfect dungeon rogue.
Evangelist- An odd, but nice niche class. Great way for the DM to turn the masses against the party.
Exotic weapon master- Great fighter class.
Ghostwalker- Just a cool class. 3.0 but it still works fine.
Orc warlord- Great NPC class.
Order of the Bow initiate- Amazing archery class.
Stonelord- A bit of an odd duck, but full of power.
Tattooed monk- One of the best, if not the best, monk PRC.
Urban soul- I feel like city adapted classes get somewhat the shaft in DnD. And this one has some unique powers. I love the concept of a "god of the cities" too.
Vigilante- DnD Batman. I can't not love it.
Zhentarim spy- Another FR class that is easy to adapt to any setting. A great spy class.
Dark Lantern- An Eberron agent of the crown class.
Some of my least favorites...
Arcane archer- Made a pretty good PRC in 3.0, but a pretty worthless one in 3.5
Assassin- For the core assassin, its just bad.
Frenzied berserker- A fairly annoying PRC to have in the party.
This thread has taught me that I love freaky badguys, secret agents, and suoerheroes.