Fee Apps (i have)
Angry Birds
Astro (File manager)
Barcode scanner - Scans QR barcodes and others
Iplayer
ConvertPad - Conversion tool
Electordroid - Electronics tool
Goggles - Point it at something, then it wanders off to find information out about it
Google Sky Map - Good for stars. Not as good as the Iphone app of similar being
IMDB - Again, handy for cheating in pub quizzes and those ' What was he in' moments
Jewels - Game
Opera Mini - Browser. Slightly faster than the internal browser
Paper Toss - Game
Shop Savvy - Scan bar code, then it wanders off to find it cheaper elsewhere (works really well)
Translate - Google Translator. Worked with a Polish friend swearing at me
Tube - Tube map and status update
Twitter - Erm..
Paid apps
Camcard - Takes a picture of a business card then creates a contact form the data. Scary how accurate it is
ISyncR - Syncs music and playlists with I tunes
Tapatalk Pro - Essential for Forum useage
Quick Office - Allows the viewing of MSOffice docs
Office Suite Pro - As above but allows basic adtiting as well
PDF reader - Erm...
Not a list of what you must have, but what i have got. Not all apps are saveable to SD card and the Desire hasnt really got a huge memory for apps, so limited.