Pageonce - Banking, bills, etc. all on your phone. This has the potential to be a killler app if they can add additional interactive functionality.
Evernote - Incredibly useful note taking via text/voice/photo. Optical character recognition and versions for web/mac/win make for a universally useful tool.
Beatmaker - Worth every penny of its $20 price. This feature-rich music composition/performance app shows how powerful the phone really is. And I get the feeling that this is only the beginning.
NetNewsWire - Very basic free RSS reader that works even when you don't have signal. Somebody will improve on this (greader + google gears, please!), but you might as well stick with this free app until somebody does.
NYTimes - It is friggin' awesome to have the entire NYT downloaded to your phone. Like NetNewsWire, it's a welcome source of information for those who find themselves without connectivity on a regular basis.