Yelp (Free)...easy to find places around you and read real reviews, makes it easy to bookmark places to try and they have a cool augmented reality function that works on 3GS phone only.
GameCenter (Free) ...newest app on my iphone taps into a massive database of video game titles.
IM+ ($9.99)...I got it on sale for $4.99. best ive found but still buggy, works with lots of services and uses push so I still get im's when the app is closed.
PS Mobile (Free)...photoshop has come to the iphone. I can't wait to see future versions.
Hold Em ($4.99)...one of the first app for the iphone and still fun to play.