Google Mobile: voice activated google searches.
Pandora: I've used it on the web for years, but the iPhone app is even better.
News Addict: I may finally be able to give up the print edition LA Times. This lays out all the top 42 news sources from NYT to CNN to Reuters to major local papers, in a magazine rack format. Tap on one, and you get their feed, customized for iPhone. Best part: it scrolls with just a tilt of the phone. Best approximation of the paper experience yet.
PUniverse. If you're an astronomy geek, this is so cool. Real time sky chart. Hold the phone over your head and it shows exactly what you're looking at. Turn, it turns with you. Tap for full data on any object in the sky. Move forward or backward in 15 min intervals, or if it's daytime, jump to the sky at nightfall. Also has separate pages for lunar phases, Jupiter's moons... I love this.
MyPhone+. With a couple of taps, grab contact, birthday, and photo from your Facebook friends profiles and sync with Address Book!
Bump. Geeky but cool. If you meet another iPhone user, you both launch Bump, and do a hand-bump to exchange contact cards.