HA! There are tons. Pretty much every game ever made by blizzard works on mac and pc, with multiplayer between the two. These include starcraft, wacraft three, and world of warcraft. For first person shooters, i woud suggest the call of duty series, the unreal tournament series, and the battefield series. Halo one is great, with cross platform multiplayer too. Lastly, if you want a fun little open source FPS, try assault cube. It is free, works on almost any machine, and has cross platform multiplayer. Any more questions feel free to ask.