... as in, "yet another freeware/opensource FPS"

this one is based on the quake 3 source that was released, so at the very least, it'll be almost as good as the original. check it out here.

Downloading.

another nice one: icculus.

and another, tremulous.

i'm loving the shit out of the q3 engine: originally designed for computers in the late 90's, it fucking rocks on middling and even poor modern hardware, without looking like complete ass (*cough* torque, the tribes engine *cough*)