Well there's AssaultCube. That's basically a super-lite version of CS. Perfect for quick games. Just google it, shouldn't be too hard to spot.
There's also UrbanTerror. Running on an engine from a *very* popular fragfest game, this is more of a "semi-realism" game which includes bandaging damaged limbs, assorted weaponry, wall-jumping, sliding on the ground and a lot more. You can also google it up, it's free and multiplatform compatible as well.
And there's also CS 1.6. Yes, there's a Mac'd version of it, but I suggest you get a licensed version before getting the Mac version, since this is basically just the Windows version wrapped in a Mac-workable wrapper.
;) keep trying!
-Anonymous