The Mac trackers tend to have Mac games. Brokenstones is probably the biggest, oldest, and best Mac tracker. Brokenstones is open to some extent: http://bswillyouinvite.me/ but invites are easy to get through a request.
There are obviously not as many Mac games out there, but I had a lot of fun playing COD2 and COD4 on my Mac for years. And Warcraft and other RPGs and Strategy games exist as well. But there are other ways to play on a Mac, like Bootcamp or playing through Cider.
