I think I'll have to start a running list of these, because the proliferation of cool iPhone apps for sailing and boating continues to astound me, and you hear about them from many places. There's no links embedded to these below, just search for them in the App store. Note, prices fluctuate.
Wind Meter: amazingly accurate measuring windspeed.
Around me: good for finding restaraunts / ATM's / beer store when you've sailed to a new port. Free.
RYA Racing Rules: summary and text of racing rules, sorted nicely.
Ship Finder: identifies all commerciall traffic (or boats with AIS transponders), including ship name, speed, heading, next port.
Compass: free app that came standard on the 3GS. Must be calibrated a bit but seems pretty accurate.
iHandy Level: turns your iPhone into a bubble-level with inclination in degrees. I used it with Compass to aim a satellite dish.
MyLite: free 'flashlight' app that provides enough close illumination to make your way across a cabin
Weatherbug: good weather app that and includes windspeed in real-time and forecasted wind. Free.
Google Mobile: a great package that also includes the Lattitude program that lets people on shore watch your progress on a map as they wait for you. It turns your phone into a transponder. This program is my favorite iPhone feature, put it on the kids' phones etc. so you know where they (their phone) is when needed.
MotionX GPS Lite: a very good GPS for your phone that serves sailing needs as well as others. Bit of a battery hog but very accurate, and free!
Wind Speed: it's an iPhone anemometer. Yeah, really, it uses the microphone.
Any others? What else is out there?