handcent sms - drop-in alternative to regular Messaging app, much better with MMS (pic mail) i think
google translate
google voice - i have my line conditional forwarded to gvoice, so this is my voicemail
google finance
google maps
camera 360 - awesome camera app with a lot of simulated SLR features/better controls
appbrain - sort of a better browser for the market, does recommendations based on your current installed apps and shows you hot apps for last 24 hours, week, etc.
adfree** - mods your hosts file so you don't see ads in the ad-supported apps. i wouldn't normally install something like this (i don't use adblock) but sometimes i find if i don't have an active 3g session that can really slow down an app.
advanced task killer - if you notice your phone getting laggy, sometimes programs improperly manage memory and don't clean themselves up.. you can kill them with this
google navigation - freaking awesome
opera mini 5 beta - some sites that don't do well on the stock browser load like a champ in here due to it's offloaded-to-the-cloud rendering model
reddit is fun - 3rd party reddit app, works well when i'm bored
shazam - when you hear a song and want to know what it is, this app can ID it in like 5 sec
tapatalk - vbulletin forum client.. requires the forum to have the tapatalk plugin installed on their side
swype
better cut - lets you create your own shortcuts on your home screen(s) (meaning you can change the text/icon/etc.. and also direct link to various programs' actions, or contacts or URLs or files or whatever)
wireless tether**
**apps = requires root to use it really