The LiveRider iPhone app and bike kit is meant to be the equivalent of the dashboard of your car, but on your bike. After going on a few rides with it, you'll feel completely naked without it.
The video explains how it works, both in terms of how it attaches to your bike, and what the thing looks like in motion. It's pretty fantastic.
I've only recently taken up biking, and having this with me lets me keep track of how far I've gone, how fast I'm currently going and even lets me keep Google Maps handy in case I get lost. You might be able to do some of these features with just using Google Maps GPS, but you couldn't track your ride to the extent that LiveRider could.
Oh, and you can even put it in ghost mode, which is you chasing your previous rides and times, kinda like a racing game.