The classic Bungie Macintosh first-person-shooter Marathon 2: Durandal is now available for free on iOS. Any iOS device newer than an iPhone 3GS will be able to experience a true classic, and the predecessor to Halo.
You can pick up the game here. The game itself is free, but offers a few bonuses via in-app-purchase like HD textures. This isn't just a slipshod port either. It offers GameCenter support as well as support for Joypad.