A computer for your bike? I know you may be thinking, "I ride my bike to get away from my computer!" Don't worry because a bike computer is just the name of a handy little device you can attach to your bike that gives you all kinds of cool info.
How do they work? A magnet is attached to your bike's front wheel (on a spoke), and a magnet pickup is attached to the fork (the bar that holds your wheel). With each tire revolution, the pickup is triggered. Based on the calibration you set for your bike (stats about your bike like tire size), the computer calculates the distance you've traveled, your speed, and other measures, depending on the sophistication of the computer.
I found one that's not too expensive called the Polar CS 100b Wireless Bike Computer ($85).
It's wireless, so you don't have to deal with securing wires around the fork of your bike. The mount attaches to your bike stem or your handlebars, and the computer attaches to the mount. It's simple to use and the large display is easy to read. Instantly, you can look down and find out how fast you are going, how long you've been riding, and how many miles you've biked.
This bike computer is water-resistant, so it can take rain and a few splashes if you are riding in puddles. It uses a CR2032 battery, which you can replace yourself. There's even a low battery indicator that'll tell you when you should replace it.
I'm a huge fan of keeping track of your workouts. It motivates you to workout and allows you to chart your progress. When I run, I use the Nike + Sports Kit for my iPod Nano, and when I bike, I use a basic bike computer like this one made by Polar.