That's really uncommon. The only thing I could find was Baldur's gate, which allows Mac and iPad cross play, but that's it. Even consoles can't cross play sadly. Call of Duty against Xbox users from my Playstation 3 would be fun, but against PC users would be unfair ( way easier to aim with a mouse.) Playstation Network and Steam are trying cross play tentatively with a game or two at a time, like portal 2. Watchdogs is a cool upcoming game that'll allow your iPad to interact and help you with the game (Watchdogs will be for PC, Playstation 3, and Xbox.)
