Another app I use frequently is a flashlight app, which will turn on either the camera flash (if you have it) or the screen to bright white. It’s great for finding door locks at night, not tripping over stuff in the dark etc.
One more useful app is AK Notepad - usually I use it for shopping lists or things I have to remember and don’t have a pen and paper handy.
Just as an aside, you have to turn on ‘Background data’ in settings when you go into the Market, and it doesn’t turn itself off automatically. It drains your battery and also allows the apps to ‘upgrade’ themselves, often in the middle of the night. Those upgrades are safe, but they add costs to your bill. Turning off ‘Background data’ when not in the Market doesn’t affect the apps or your phone at all.
I also frequently use Advanced Task Killer and SanDisk Memory Zone for maintenance.