My Personal List:
Pandora Radio
Shazam (just cool, download it)
Zedge (Wallpaper and Ringtones, all free)
ASTRO File Viewer
Spare Parts
Handcent (alternative SMS App)
Advanced Task Manager (I prefer to Task Killer)
Google Sky Map
Weather Bug
Swype (can't live without it
http://androidforums.com/htc-droid-eris/51161-swype-trial-period-ended-wtf.html#post743091 click the link for a working download link)
Barcode Scanner (will let you use your camera to scan barcodes like in the Swype link as well as any others)
Bonsai Blast (cool game)
Abduction (funny game)