I am in love with my iPhone. it may become a problem, but for now, I am utilizing it for all that it's worth. one of my favorite apps right now is the iMapMyRun application. I've been using the site for years to plan out my runs around Pittsburgh, but the app is extremely useful. it acts as a GPS and stat recorder. it gives you pace, distance, and has a map that can help you out if you get lost on your adventure (as I often do).
I will often just go out the door without any set goal or route, but it's nice to be able to go wherever I feel like going but still know how far I went once I return home.
I usually run with my iPod shuffle, but running with my iPhone lets me use this app and take spontaneous pictures. you never know when something is going to strike your fancy.
do you have an app that you can't live without?