Joel Rosenberg's Blocked is currently the #2 paid game on the App Store (behind iDracula), and STP can see why. It's functional, dirt cheap, and it has 100 smartly designed sliding block puzzles to work through. But it's also missing a way to measure your skill, and that's sure to turn some players off.
The sliding block puzzle is one of the oldest, and best, tricks in the puzzle game book. The idea is to slide tightly interlocked blocks (some vertically oriented, some horizontal) back and forth across a square level, freeing up a path to help a target block escape.