iOS Game Engines
Perhaps the most widely-used of game engine for iOS is cocos2d for iPhone. cocos2d has a long feature list including OpenGL ES-based graphics, integration with the physics libraries Box2d and Chipmunk, a particle system, scene management, effects *such as waves, ripples, etc), sound support, tile maps, and more. It also is adding some support for 3D graphics. cocos2d is also now available for Mac OS X, so you can deploy the same game made in cocos2d on the iPhone, iPod, iPad, and Mac. One other benefit of cocos2d is that there are already a few books written specifically using it to create games for iOS.
Other game engines available for iOS (in no particular order) include: