I've avoided this thread for far too long. I definitely have an app-addiction.
I'll second Tweetie, Mint.com, What's On? and FourSquare
and add:
UPS
This American Life
Enigmo - This game is fun but I am terrible at it.
DoodleJump- Super addicting game.
Gowalla- Like FourSquare but prettier.
Yelp
NYTimes
AP Mobile
NPR
BeejiveIM- For ALL you instant messaging needs.
IMDb
Fandango
Urbanspoon- I haven't found a good restaurant on here yet but I like the slot machine noise it makes.
OpenTable- Makes dinner reservations
SimCity
Skee-Ball- Another really addicting game I'm terrible at.
Siri- Finds restaurants, movie theaters, movie times and so much more. It's like your personal life assistant.
And if you're into that sort of thing:
iReddit
Shovel (Digg)
TWiT