For me my top 10 most used apps would be:
Jailbreak required | App Store
1. Echofon (Twitter app)
2. SBSettings (access all important toggles quickly)
3. Lockscreen Info (put my calendar on the lock screen)
4. Remote (kind of obvious: it controls my iTunes)
5. Scrobblr (Scrobble straight from the iPhone)
6. Byline (RSS Reader)
7. Weather Pro (It's all in the name)
8. MobileFotos (Manage and view my Flickr account from the iPhone)
9. AptBackup (saves a LOT of time after upgrading the OS)
10. BiteSMS (Replacement app for the default messaging app)
There are plenty more of course, but these are the ones I use most
