Tremulous is a very fun fps. It is opensource, based off of Quake3, and can be quite addictive. Players can be humans, and play the traditional fps point and shoot method, or they can play the aliens, and have to learn all new tecniques, but have very powerful closerange attacks. Or players can build structures for each team's base, required to succede.
Check it out:
Main site, with downloads to Windows and Linux versions
Modders site, provides a mac version, more secure clients with GUID (required for some server's admin interface, aka for a player to be an admin, they need a guid), and an admin interface for servers:
Mac Version:
Aqua-Soft Forums