1. IM+ Lite -- great IM aggregator app. I'm using it for AIM and Gtalk, but I think it can do Facebook chat, Yahoo, and others. Provision for push notifications so you can get IMs when you're in other programs. Free.
2. Facebook -- iPhone app, so renders small on the screen, but works well enough. free.
3. NPR -- can listen to most stories and read all. free.
4. Kindle -- of course. free.
5. Dictionary.com -- everyone should have a dictionary handy all the time. This app is a fantastic implementation that works offline. With an Internet connection, you can play phonetic pronunciations of the words -- great for people like me, who grow vocabulary by reading rather than hearing the words spoken. free.
6. Penultimate -- the only app I have paid for. Notebook app that allows for drawing with fingers. Great for meetings and diagraming ideas. Can email images. $2.99
7. AccuWeather -- beautiful app that provides weather quickly and unobtrusively. free.
8. NY Times Editors' Choice -- nice summary of the times in a paper-like layout. free.
9. Netflix -- killer. I can watch any of the "Watch Instantly" content on the device. Looks great. free.
10. OpenTable -- make restaurant reservations on the go. iPhone app. feee.
11. Shakespeare -- collected works, beautifully formatted, unlike some public-domain Kindle works. free or $20 pro version. I may buy the latter just to support the project.
12. Popplet Lite -- good for memory maps and diagraming thoughts. free.
13. Shazam -- IDs songs that are currently playing using the built-in mic. free.
14. Wikipanion -- Nice Wikipedia app. free.
15. Blackboard Mobile -- so I can access my course site on the go. free.
And, of course, I use the built-in apps: Mail, Safari, iPod, Notes (I like this a lot), etc.