Cocos2d iPhone - This is a framework for building 2d games based around OpenGL ES 1.1
Sio2 Project - This is an open source engine built for 3d games and is also based around OpenGL ES 1.1
Chipmunk Physics - A complete physics suite, it is built into Cocos2d, but can also be used outside of Cocos2d as well
Unity3d - I've never had any personal experience with this, but everyone I've heard from who has has loved it. They have a free trial, but the full program is quite expensive (at least for me!)
You might have noticed that each of these seems to be geared towards game creators. I can see two likely reasons for this. The first would be that games seem to be doing very well in the App Store and many people would love to get involved with them. The second would be that games can be one of the most difficult areas in programming and these can be used to simplify the process and reduce redundant programming.
You may not find these tools useful for yourself, but I have experimented some with each of the top 3 and can see the merit for any of them. I will probably not cover much on these tools beyond this post, but felt that iPhone developers may want to know about them.