My friend Scott let me know about a new app/service called Strava that uses GPS devices to track your rides and runs. I've been using the Nike Plus GPS app for runs, but have wanted something like it for riding.
The iPhone app has a really clean interface and seems to track remarkably well. I have used Mapmyride in the past, but it was never very accurate.
One of the killer features is that it allows you to create "segments" from your rides and then when you or others ride it, your time is recorded on a leader board. It can be helpful for tracking your own progress or for competing with others who use the app.
I rode my fastest commute to work this morning in a long time.
The app is free and pretty full featured and they do have a paid premium membership with more features. You can also use other GPS devices or Android phones. There is a good web interface, which is nice for viewing maps and looking around more.
Enjoy!