Cube 2 - New fun "quake-clone"
I just downloaded and ran Cube 2 and its very fun.
Apparently its a new game-engine written from scratch and its really nice.
Some maps are made after popular CS-maps and Quake-maps and its a very simple but fun FPS-game.
They have native clients for MacosX, Windows and Linux and it works just great.
Try it out!