LeafSnap - take a picture of a leaf and get it identified. I haven't been able to test this much since we have no leaves yet! But I love the idea. Only some US trees so far but more promised.
Photosynth - helps you photograph panaramas, stitches them together, and lets you view them. From Microsoft.
A part of me would like to escape the Apple closed world, maybe buy an Android tablet instead of an iPad. But I really like the huge variety of apps. Some of them come out on Android as well, but not all of them.