This $0.99 application allows you to record a track on your iPhone 3G and then upload it to the Path Tracks site. Once on the site you can share your paths with others or export the paths to GPX or KML and then use them with Google Earth or other programs. The program also gives you the total distance travelled, the time taken and your average speed, so you can easily use it to track your exercize progress. I now leave my Garmin Forerunner GPS watch at home and just carry the iPhone.
Above you see a track of a walk I recently took. You can find all the details about Path Tracker here - and for $0.99 you can't go very far wrong.