It comes down to what games you are running. If you are running a Source engine game(like Portal) all you have to do is enter "cl_showfps 1" in the command line or "net_graph 1" without the quotation marks(set 1 to 0 to get rid of it). There's no equivalent to FRAPS on OS X to my knowledge in which you can see the FPS in every game so you'll have to do it on a game by game basis.