Game Engines

cocos2D for iPhone

http://www.cocos2d-iphone.org

Sparrow Framework

http://www.sparrow-framework.org

SIO2

http://www.sio2interactive.com

Unity 3D

http://unity3d.com/#iphone

Unreal Development Kit

http://www.udk.com/mobile

iTorque2D

http://www.garagegames.com/products/torque-2d/iphone