Tomb Raider Anniversary(not online, but why wouldn't you want this game.).
TrackMania Nations Forever(completely free racing game, really good graphics, very competitive), but you will need to bootcamp it.
I realize you keep saying you don't want to bootcamp, and I am with you all the way, I hate using windows as well. But if you want to have a wider variety of games to play, then you will just have to do it.
If you do decide to bootcamp, then check out Team Fortress 2, Counter Strike: Source, and a half life 2 mod called Insurgency, all are a lot of fun. And keep watching Steam for weekend deals and package deals, they have them all the time with really good prices, sometimes they have games as low as $5.
If you are not gonna use bootcamp, then let us know when you find a good online mac game, I am sure there are some out there, I just haven't seen any yet.
Once COD4 comes however, we will get what is probably the best online game out right now.
You might try COD2 or UT2004 if you want something now, there seems to still be a few people that play them, each are about 20 from amazon.
And finally, you could always get Quake Wars, it is online, and it looks good, I don't know how great the gameplay actually is. I think you can get it for like 35-40 off amazon though.
