I am an Apple junkie. Here are a few I like:
Harry's Laptimer and a hi-res GPS receiver is a must have if you are a performance driver that runs a lot of AX/DE/TT. Here is a list of recommended GPS receivers, motorsport quality mounts, and OBDII devices that are compatible with iphone and Harry's.
http://www.gps-laptimer.de/Compatibility.html
Mapquest- Way better than the Apple GPS mapping IMO with live traffic updates.
Google Earth- Amazing function that does some things better than Mapquest.
Scanner Pro- A photographic scanner that sends documents anywhere with a few clicks.
Amazon- Quickly get current best pricing on anything, anywhere. Buy local or order online and wait a few days for it to be shipped. You decide with info at your fingertips.
Tons of very useful music/audio related apps if you have an interest in that.