This isn’t a definitive list, just a personal one. I’ve recently re-flashed my phone so it’s time to install my favorite apps on it.
Free:
Handcent SMS
K9 Mail
QR Droid
Applanet
Battery Indicator
Wordpress
Pandora
Tweetdeck
Zeam
Quick Settings
Soundhound
Advanced Task Killer
Astro
Sketchbook Mobile
Paid:
Chandroid
Tapatalk
PicSay Pro
Hard to find:
Swype
Posted by Robert 
