System Activity Monitor-- especially in a 3G it's important to have a way to free up your memory after playing a game or something
WiFiTrak-- to find the best wifi reception
A battery monitor app (there are a hundred to choose from)
MoodAgent-- makes playlists from songs on your phone based on criteria like speed, joyfulness etc, much more fun than Genius
Parachute Nija-- just a cute, fun game
Voices-- a voice-changing app that again is just fun and cute.
Vlingo and Google App-- for voice commands and voice searching that isn't native to the 3G
Qik Video Camera-- to take video clips with sound
PhotoForge-- picture editing right on the phone
iLoader for Facebook-- uploads multiple pictures and handles photo albums, something the FB app can't do
Inspire-- painting on your iPhone
NewsAddict-- lots of news mobile sites in one app, from MSNBC to GoogleNews to CNNMobile
Enjoy your phone