Barcode Scanner Get this first - it's your gateway into grabbing other apps. Point it at those funny square barcodes used on Appolicious (and elsewhere) and you'll be automagically sent to the download link of the app. Really slick.
Twitter Not as nice as the iPhone version (what, no multiple accounts?) but it still gets you rolling with the obligatory 140 character messages.
Facebook Also not as nice as the iPhone version, but passable.
Pandora® internet radio This personalized-music service is awesome on any device. Haven't tested it over a 3G connection yet, but works great on WiFi.
Dropbox Must-have to get important files on to your phone (the Android OS doesn't make it easy, especially if you have a Mac).
Camera 360 Free version offers some nifty toy camera effects.
Google Voice I still have my iPhone kicking around, so I need to use Google Voice to ring me no matter what phone I happen to have. This app helps you manage that connection as well as set up voice mail on GV (instead of your carrier).