ngmoco's recent winner of a 2009 Apple Design Award, Topple 2 [App Store] is currently on sale for 99¢. Topple 2 takes the original (no longer available on the App Store) Topple formula and adds additional gameplay modes as well as WiFi multiplayer an email challenges.
From my review of Topple 2 a few months ago:
The premise of the Topple series is simple, you stack blocks of various shapes and sizes to reach a set altitude goal within the provided time limit. Blocks can be rotated to fit better in to the existing stack and the game makes excellent use of the accelerometer. If your tower of multicolored blocks isn’t as sturdy as you would like it to be, tilting the phone left or right causes your stack to lean, potentially giving you the extra bit of stability you need in order to not send everything crashing down.
Mostly due to WiFi challenges, both Topple 2 and WordFu [App Store] have lived long and happy lives on my iPhone. If you're looking for a colorful stacking game filled with smug looking squares and menacing triangles, give Topple 2 a try. The latest update included iPod controls, and with any luck, Plus+ integration will soon follow.
App Store Link: Topple 2, 99¢