I recently received my backflip in the mail from amazon and I'm enjoying it. I'm a current 3GS user and I like mixing things up with exploring the android platform. I wanted to know what signature apps I should download : )
Texting - Handcent
Email - K9 Mail
Astro file manager
Aurora search
Opera Mini
Ringtone Apps
Ringo Lite (set custom text msg tones per contact)
Rings Extended (pick full songs for ringtones)
Ringdroid (cut sections of songs for alert and rintones right on your phone)
Neat but somewhat less useful - Compass and Speed proof (speedomter), WiFi Analyzer
aCar - if you want to log your vehicle expenses
SlideIt or ShapeWriter (virtual keyboard replacements)
Playlist Builder - to build music playlists on your phone.
If you use ActiveSync (exchange), TouchDown is much better than K9 for that. It syncs your Calendar, Contacts, Email and Tasks.
I would also recommend the following:
Bluetooth File Transfer
Astro - File/App Manager
Any Cut - Allows you to create a shortcut to just about anything
PicSay - Pretty cool Camera/Photo app
Pandora - Online Radio
WikiMobile
Wifi Analyzer - I'm a techy so this is a cool app for WiFi