Any iPhone owner's worst nightmare is forgetting his/her phone at home. It's really amazing how that damned device manages to affirm itself as a necessity for everyday life, and how people freak out when they somehow forget it. I know for a fact that ever since I got mine about a year ago, the longest it's been out of my reach can be calculated in hours, if not minutes. When it's not feeding me some music through earphones, it's in my pockets at all times, ready for duty.
That being said, it's only natural of me to bring it along when I go running (and trust me I've been doing an aweful lot of that lately). It used to be only to play some music, but about a month ago I found an app that wonderfully compliments my jogs: Runkeeper. The concept is pretty simple, and it's been done before numerous times: the app is basically a tracker that records your position at regular intervals using the iPhone's built in GPS, then compiles your distance traveled, average speed, max speed, and a couple of other stats. What makes Runkeeper different is the really nice web-based backend that allows for much more thorough processing of your record runs to extract more significant data and giving you the option of posting your runs/hikes/rides onto Facebook or Twitter to inspire your friends to get running as well.
While other apps only calculate distance, time, and speed without giving you any additional info, Runkeeper actually takes that data and gives you all the web2.0 goodies, like embeddable maps, all sorts of crazy charts for stuff like vertical ascent/descent per km, a speed/time bar graph, and even a calorie counter, for whomever that may interest. It's more than a dumb bike/run computer, it's a complete fitness journal that acts as other training guidelines and motivator. Plus the best part is that it's free, although there is a premium version that isn't ad supposted, and has even more interesting features like audio cues as well as in-app training programs for those of you who are totally clueless about running. The premium version is only 10$, which is way less expensive than the Nike+iPod system (29$ + shoes), it's primary competition. No need for special shoes, and you don't need an iPhone 3G S, it's compatible with either 3G iPhones or 3G S models.
All in all, I only have on complaint, and it's that you cannot edit all the info available on the website directly from your mobile device without having to fire up Safari and navigating through a very phone-unfriendly site. Other than this minor issue which is most likely to get fixed via an update, you have no reason to not try Runkeeper out. The chances are that if you're a regular runner/biker, you'll be getting the Pro version in no time.
Just don't forget about the iPhones moisture sensors, and always use and adequate holster or a ziploc bag while running!