Must have...
Seesmic, of course, is the best for Twitter.
Apndroid, becouse battery life is not a strong point in our beloved Droid.
BeyondPod, simply THE podcast manager for Android. I bought the full version, and I'm quite happy.
FBReader is amazing, just the best ereader I've used (and I'm an avid reader wich doesn't read a paper book for many years).
Astro File Manager, a free app that does it all.
PDANet, another freebie that's saved me a lot of money - no extra bucks for navigating with my notebook!
And, of course, the most important app, Dolphin Browser.