I use RunKeeper. I actually don't carry my iPhone with me when running (it bothers me), but I do when cycling. So when I run, RunKeeper lets me manually click through my route (on the web only currently); when cycling, it does a great job tracking my route.
I find that having a record of all my runs motivates me to continue. I also tend to post my runs to Facebook; it makes me feel like others will appreciate my hard work :)