Barcode Scanner - To scan QR codes
Quick Setting - To turn off 3G
LockPatternOnOff - To one-click enable your lockpattern
AppsInstaller - FAST way to install any app that you may have gotten from a non-market (i.e. LEGAL) source
HistoryEraser - Clears call logs, browser history, market searches, etc
Ringdroid - Make your own ringtones, alarms, notification tones from music on your SD card
SysTray Monitor - Displays percentage of battery left on the notification bar (and/or a few other things)
Google Voice - ... if that's your kind of thing...
There are a few others too, but I'm not sure what you use your phone for so they may be pointless
If you are going to root (get admin controls of) your phone, there are a ton more!
Welcome aboard BTW!!