These aren't "must have," but they're a few of my favorites.
Alarm Clock Plus - you can set it up to have you solve a math problem to be able to snooze or dismiss it. It's great for me because I'll his snooze in my sleep.
AppBrain- you can go ahead and go to
Top Android Apps and Games in the Android Market | AppBrain.com now and start making a list of apps you'll want to download. Then when you get your TB, download the app on there and sync it.
Backgrounds
Better Keyboard
Phandroid
SiMi Folder - just found this and I love it! I hate having a cluttered home screen
Handcent
Loot - checkbook app
Photobucket app - you can have it automatically upload your photos as you take them. It's nice for a backup.
QuickPic - it's an alternate gallery. I just found this one as well, but I love it!