I don't use my iPhone a lot, at least when I'm at home. When I'm "out and about" in general, I use it mainly to kill time while waiting in line somewhere. In normal usage:
Mail
Safari (web browser, mainly for PF and a couple of other forums)
Camera + Photos (I'm counting them as one entry because they go together so closely)
Weather
Stocks
When I'm traveling like on the road trip I did last May, I use Google Maps more, of course, and I do a lot of general Web stuff like searching for sightseeing information and booking hotels, because I no longer take my MacBook (laptop) along on those trips.
Special case: when I visited Portland a couple of summers ago, I used the Tri-Met (transit system) app a lot. Their "next train/bus arrival time" feature was very useful for telling me how long I'd have to wait for my next chance to take a picture of a train.