Urban Terror
a free game, about a 700 MB download, used to require quake 3, but no longer. runs great on os x, i get smooth 60+fps on my macbook pro.
Not really a single player game, but great to go fight some people online easily. Free for all, Capture the Flag or bomb mode are the best depending on what kind of action you're looking for.
http://www.youtube.com/watch?v=KQPhpBdGnXY
my favorite parts are the physics and how you can strafe jump (bunny hop in an arc) to get places quickly, knife and boots are fun to humiliate or if you run out of ammo, and if you can jump on someone's head from 10 feet up or more they get curb stomped.
there are lots of maps to test these jumping skills, and running in arcs on ice makes you go really fast
jumps:
http://www.youtube.com/watch?v=MTnte8DnyZQ
icyjumps:
http://www.youtube.com/watch?v=wIp02sZ6cBc
icy ones get pretty out of control by the end of the vid