iKids Puzzle is a really cool tangram puzzle app for the iPad that is free right now. (normally $1.99) I was lucky enough to pick this up for free about a year ago. It has a cute (clearly Asian influenced) user interface. It is a fairly forgiving puzzle app since your child just needs to drag the matching shape to the outline. The app will correctly position the shape.
The puzzles are tangrams of numbers and letters which is great reinforcement while having fun with the puzzle. M just loves this puzzle game and giggles at the funny expressions of the shapes as they fall into place.
Highly recommended for ages 2 to 5. Grab this deal while it lasts.