RunKeeper is an iPhone application that enables you to use the built-in GPS in your iPhone 3G to track all of your outdoor fitness activities, including duration, distance, pace, speed, elevation, calories burned, and path traveled on a map. You can store all of your historical activities on the RunKeeper website and share them with your friends!
The better news? It is available in two versions: a free version (fully functional) and a pro version with advanced functionality.