Codea is a Lua IDE for the iPad that is especially useful for creating games and simulators. It includes syntax-highlighting and visual editing for Lua, a graphics renderer, reference documentation, shader support, a Physics engine and support for touch, accelerometer and camera. Finished Codea apps can be exported to Xcode and built as App Store apps.
Bear in mind that Codea only supports Lua, not any other languages, and that it’s not a substitute for Xcode. If you have a “see what works” attitude and a willingness to read the Codea forums, you may pick up Codea and Lua quickly; if you expect extensive tutorials, you won’t find them here.
Codea, Two Lives Left, $9.99