My list of apps installed is quite large, however my list of "I use this all the time" is not as large.
1. Safari, Mail, iPod, Video (all built in, so listed together)
2. Feeddler - I follow some 30 RSS feeds and this app is awesome. Flipboard just got updated to support Google Reader feeds, so will check that out today.
3. Twitter
4. The Weather Channel MAX+
5. DocsToGo Premium (gotta have the Cloud access!)
6. Netflix
I use others, but the ones listed above are used every day.
Also, to AlunReynolds - DocsToGo Premium allows direct access to a large number of Cloud Services: Dropbox, Box.Net, Google Docs, iDisk, Public iDisk, SugarSync. It allows the sharing/syncing over wifi with your PC as well. I keep 4 devices sync'd using Dropbox and DocsToGo.