Shooters
If you like shooters, there are two free ones I highly, highly recommend:
Wolfenstein: Enemy Territory:
http://www.planetwolfenstein.com/files/files.shtml
True Combat Elite:
http://www.tcelite.net/r
Both of them are COMPLETELY FREE and have a wide base of fans. Enemy territory plays a bit like team fortress 2, except the setting is world war II. Its class-based play, so you choose to be a medic or a soldier or an engineer or a covert ops. Each class has strategic special abilities, and the maps are objective based. For example, in some maps you have to blow up an objective, like a gun encampment, and other maps you have to drive a tank to a a bank and steal gold out of the vault and get away with it. In fact, the "goldrush" maps in team fortress II are based off of the "goldrush" map in wolfenstein. When world of warcraft 1st came out, wolfenstein: enemy territory was the only game that could draw me away. You play as the axis or the allies, with allies usually attacking and axis typically defending.
True combat elite is a mod for enemy territory that changes the game completely. Its a lot like counter-strike. Basically, you get 1 life, and rounds last something like 5 minutes. There are classes too, but its pretty much a choice between whether you want an assault rifle or a sniper rifle. In this game you move very slowly, and have to stop and aim down your iron sights to shoot someone. You don't have cross hairs to aim, so you have to use your iron sights. Typically the terrorists in this game are trying to plant a bomb, and the "good guys" want to stop them.
So yeah, they are both very different games, but I have had a lot of fun playing them. They're both also cross platform, so you can have lan parties with your mac/pc/Linux friends and play online or against each other.
Also, you borrow unreal tournament (the original game, not 2004) from a friend and install it on your computer. Online play is free, and you don't need to authenticate the game or anything. Just install it and play! There are TONS of mods, and its still a solid multi player game. So if a friend has the game and never plays it, convince them to let you have it.
wikipedia also has a decent list of free games:
http://en.wikipedia.org/wiki/List_of_open_source_games
http://en.wikipedia.org/wiki/List_of_freeware_games
http://en.wikipedia.org/wiki/List_of_free_first-person_shooters
http://www.dmoz.org/Games/Video_Games/Downloads/Free/
Some of which (like enemy territory) are available for mac.
The take home message: Wolfenstein: Enemy territory and the original unreal tournament are pieces of game history. Both games are lots of fun, and if you haven't tried them out, you SHOULD! On new macs, you can play them with all the graphics settings maxed out, and things actually look pretty good! Wolfenstein is STILL the only shooter I regularly play.
