Well, we've had them for a couple of weeks now, so what have you put on there that is 'essential'?
Me, I currently have 3:
1. Dolphin Web Browser (free). Much better than the default browser and supports multi-touch, tabs and gestures.
2. SmrtGuard (free). Remote back up, tracking, call logs and data wipe, all in a nice package.
3. Better Keyboard ($1.99). Does what it says on the tin.