I say download everything you think you might like, try them all and delete what you don't want. Shopsavy is great if you have ever been shopping

it is one of the two biggest reasons I got my android phone the other is sharing calendars with my wife. The program advanced task manager is good to help uninstall quickly too.
Here is my list so far.
Facebook
Keeper for passwords
Note Everything
Battery Life
Samba Explorer to get files from your home computer via wireless
AndroZip for file browsing and zip files
PhoneFlicks for Netflix
K-9 email if you want to be able to change your reply address
Banking apps like BofA
Gem Miner game

Power Manager to help save battery
Speed Test to see what kind of network speeds you get wireless/EV
Unit Converter
Bluetooth file transfer
Spare parts for more settings - all volumes
Ringtone scheduler so your emails all night won't keep waking you up
Ring droid
MyBackup to backup to SD card
Battery Graph
Call Location so you don't have to guess 100% on area codes
Color Flashlight
Handcent SMS because it has a popup so you don't miss messages
Mortgage and car loan calculators
Movies
Scan2PDF is useful
Shazam is neat
Thinking Space for mind maps
If you find better let me know. I would really REALLY like an app to allow snoozing individual calendar events.