Google Skymap has got to be the coolest app I have ever used. Hold it up to the sky and it has the stars and planets labeled as you see them.
TouchDown works great as an Exchange/Active Sync program (way better than the OS email/exchange app.)
I know it is part of the new OS, but the Google Maps Navigation is also really slick. I just got a MotoDroid along with the car cradle and I just drop it in touch the voice search button and say "navigate to Piggly Wiggly" (grocery store) and it brings it up on a map for me to verify the destination, then I can select navigate and it starts mapping and telling me how to get there. It even reroutes as soon as it realizes that you have strayed from the original route and when you arrive, it switches automatically to street view so you can compare your surroundings to a 360 degree on-screen view.