I have no idea what this thread has to do with Mac lovers, I have an iPhone and don't have a Mac.....
Dynolicious is worth every cent. Its very accurate (as has been proven time and time again), and gives you all sorts of different stats.
Myst for iPhone is a direct port of the original Myst, very intuitive to play with a finger, and after I bought it I couldn't put it down.
TweetDeck if you use twitter is a great app. It uses columns to organize everyone's tweets.
PaperToss is a fun game.
Wolfenstein3D is tons of fun, direct port of the original, but maybe slightly sharper sprites.
Skype for iPhone is exactly what it sounds like, comes in handy if you use Skype regularly.
Pandora Internet Radio is the one I use the most during the day at work.
You can also jailbreak it and have access to a whole world of Apps that, for one reason or another, were not allowed on the app store. The one I get the most use out of is Flashlight, which used to be on the app store, but got taken down because it hacked the brightness of the phone so that it would put out more light. Super handy.