If you're in to FPS's, go download OpenArena (it's free). It runs really well on my MB (same model as yours) at good settings, and there are plenty of players online most of the time. It's based on Quake 3, so the graphics are nothing spectacular, put it's fun.
Lots of games will run if you don't mind low settings and OK-to-bad performance. These include:
Civ IV - you pretty much need to put every graphic option down to the bottom. The game will run great for a while, but in the later stages your framerate will start suffering badly and it'll take FOREVER to go from turn to turn. I still play all the time, however!
Spore - With the latest update, everything works (once again with all settings down). Creature stage will run decently if there aren't many creatures onscreen, and the framerate will slowly degrade as the game goes on. Definitely playable though.
WoW - I don't play, but I know it works!
Age of Mythology/Age of Empires III/C&C Generals - Yeah, they're old, but they play just fine!
If you've got Boot Camp running, you'll be surprised to know that Rome: Total War works totally playably on low/medium settings. Definitely check it out!
Disclaimer: I consider all of these games "playable" because I'm a lifelong Mac user (and mid-level Macs at that), and am therefore THRILLED when I get a game that runs over 30fps consistently. If you're coming from a PC gaming powerhouse machine, you'll probably end up tearing your eyes out!