Isaac Newtons Gravity HD - iPad Apps for Education

This is a really well designed application that requires logic, problem solving, hypothesizing, and testing in order to complete the puzzles.  The demo version (free) gives you access to 5 puzzles, but I was instantly addicted and paid $6.99 for the full app which features 100 puzzles plus the ability to create and share your own puzzles.

Isaac Newton's Gravity HD

The basic idea is that you arrange objects provided at each level so that a chain reaction causes the red button to be pressed.  When you hit the play button, a marble drops out of the opening in the upper left and proceeds based along the path that the board provides.  The design environment is really open-ended and the satisfaction of a completed puzzle is great!