I only have an iPod Touch, but I've found some great apps too.
1. CubeRunner (free). Very simple but VERY addictive game that uses the built-in accelerometer. This was actually the first ever game I got from the App Store.
2. Labyrinth LE (free). It's a limited version of the $$ full version with just 10 levels but it's great fun.
3. StarMap. (£6). Bought this on the strength of Bruce's recommendation and also because I have this hankering to find out more about the constellations and the planets. Like Bruce, I've not yet used it in earnest.
I've got a few others that are OK but I might end up deleting them soon as their novelty wears off.