Man, most of the games I play on my Mac are freeware.
*Macswear: Mostly because of the awesome name, but the old-school 3D gameplay is still fun.
*Amoebax: A Puyo Pop clone.
*Bob's Dream Lite: There's a paid version on the App Store, but not on the Mac.
*Bullfrog: A cool arcade game to have fun with every now and then.
*Coblyn: BOULDERDASH!!!!!!!!!!!!!
*Supertux: Best. Linux game. Ever.
*Jumpman: A truly revolutionary platformer that you should check out.
It's an oldie, and a demo, but free! The Medal of Honor: Spearhead multiplayer (2 maps only) demo is still great and there are still a few servers going that you can play on. Being an oldie, you should hopefully be able to play on low grade tech
It was my first big push into online gaming, clan matches etc etc. Convinced me to buy the full game and had me hooked for years back in the day... ah, the good old days...
I'm really into OpenArena...it runs on the ioQuake3 engine and is completely open-source...personally, I prefer it to similar games like Urban Terror and Nexuiz, but that's just me.
Rock it on the iMac, instant kill with railguns...that's me...
Benn playing quake live the last few days, and its really good!
And free!
Installing the first plugin and stuff did take less than 5 minutes and the game just installs and download what you need when you play thru the tutorial!