The fantastic Nike+ App let you track how far, how fast and at what speed you're running with only a sensor in your Nike-branded shoe and a compatible iPod or iPhone. Now you don't need the sensor or the shoe.
The $1.99 app, which just got posted to iTunes, uses GPS to track where you run—something the old version was lacking—but still includes the community features that made running not quite such a lonely affair. Now you can use the iPod touch 2nd through 4th gen, and the iPhone 3GS and iPhone 4 with the Nike+ app. Only the iPhone works with GPS functionality unless you have something like Magellan's iPod touch case, which adds GPS ability (along with a lot of weight).