If you are developing an interactive 2D application for the iPhone or the iPad I would like to recommend the graphics library Cocos 2D. We recently used it in the Saab 9-5 applications for iPhone and iPad, and Cocos 2D is easy to use, robust and has very high performance. Other well-known applications using Cocos 2D include FarmVille by Zynga and The Elements: A Visual Exploration.
The best thing about Cocos 2D however is that you can use it completely free of charge. All you need to do is to acknowledge that you have used it in your product. Check the about box when you’re placed in the Gallery section of the Saab 9-5 applications for iPhone and iPad and you’ll see it there as well.