The fantastic Nike+ App let you track how far and how fast 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 $2.49 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).
As a fan of the original Nike+ concept, I'm happy that this lets more people running in order to make it more likely that I have someone to challenge to a race. [iTunes]