My favorite app by far is "Air Video". It allows me to stream all my Divx movies to my iphone over 3G from my PC at home. That used in conjunction with "Logmein" allows me to download new movies/TV episodes when I am away from home.
Pandora
GPS Drive by MotionX
Trapster
My Radar & Weather channel
Grocery IQ - Simply awesome if you are married and both have iPhones.
Whatsapp
Red Laser
Quick Tip
Abby Business Card Reader
Tune in Radio
Shazam
Lyric Find
Photoshop Mobile
Transfer - wireless transfer of movies and photos
System Activity Monitor
Alarm Clock
Meebo is the only app I can suggest without repeating anyone else (I think...). It's a nice, FREE multi-client instant messenger app. Only thing is you need to register for a free online account.