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Besides stock apps (mail, Safari, iCal, Google Maps, Phone, iMessage, Music)

Nightstand Central
Navigon
Evernote
Dropbox
SugarSync
Speedtest
Flashlight

(More on iPad NY Times Crossword Puzzle, Kindle, iBooks))

(Only on iPad: Scrabble, iAnnotate)
 
Just stumbled across Dragon Go. Pretty sweet for those of us with an iPhone 4, without Siri.
There's also Vlingo, which I believe is now owned by Nuance as well (the makers of Dragon apps and essentially the backend of Siri).
 
Not counting default applications: I text and chat a lot, so FaceBook Messenger, Yahoo Messenger and Skype are on my top list. Aside from that, I am addicted to 9GAG, so here it comes, the dedicated 9GAG Reader :p.
 
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