My top 20 favourites are:
AudioManager -- to allow volume control of everything (particuarly useful for alarm volume)
ActionComplete - Task/project management (or use Astrid or GTasks)
Aldiko - EBook library and reader
Age of Conquest - Risk type strategy game
AppManager - Backup and install apps (such as those not in the Market)
Astro - FileManager
Bonsai Blast - Game
Google Sky Map - AR star chart
Handcent SMS - iPhone like SMS app
Mobisle Notes - Note taker
Movies - Trailers and what's on
NetCounter - Keep track of data use
RoboDefence - Game - but not in the market need to search for the apk online
TriCorder - Sensors data. Mainly Just for fun
Underworld - Game
Well, that's 15 of them. The five others are UK relevant only, like train times, last.fm and TV guides etc. Of course there are the built-in ones too, like maps and gmail, but I've not included those here.
Enjoy.
- Mark
EDIT: How could I forget - SHAZAM!