The one I was showing them today is free--the best kind--and it is called Montezuma Puzzle. It is similar to tangrams. You have to try and fit specific pieces into a given frame. It really tests spacial reasoning. There is a volume two if you master the first. It is one of those addictive apps!
What apps are you using to entertain and educate? We do have a class iPad, so I'm always looking for good, engaging, and relatively inexpensive apps to use with the class. Please share!