If you're into fitness, and like playing with technology too, there's a great assortment of computer watches you can use that allow you to track your fitness. Because technology continues to be miniaturised manufacturers can now put everything from heart rate monitors to GPS sensors in a regular sized wrist watch. And yes, these devices tell the time as well.
There are two main categories of sports computers - those that you wear on your wrist, and those you attach to your bike. If you're an occasional cyclist, there are watches that can be mounted to your bike, but they don't provide as much functionality as specialised bike computers. For a good general sports watch check out the Polar and Suunto ranges. Both companies have watches that give you basic heart rate information from under $200. The base models (the Polar WM21 and Suunto t1) also let you keep track of your fitness by providing in-built fitness programs. The Polar WM21 for example has a personal weight management program, which helps you to lose and maintain your weight.
For more advanced users there are very sophisticated watches like the Polar S725x and the Suunto t6c, that, as well as providing basic heart-rate functionality also come with an altimeter and can be customised to adapt to different conditions. The more sophisticated ranges can also talk to other sensors, like a foot sensor that can measure how far you've run, and a bike sensors that can measure your speed and cadence. In Suunto's case, there's even a GPS sensor (called a Pod) that can talk to the watch. These watches generally start above the $400 range, and can cost as much $1000, once you factor in all the extras.
If you're a serious cyclist there are also custom computers designed especially for the bike. Garmin, for example sell a GPS bike computer that measures your speed and distance via GPS satellite. The latest model, called the Edge 705 even has a colour screen to show you where you've ridden, and different modes that help you train better.
Once you've finished your exercise, most advanced devices will even let you download your data to the computer for archiving and analysis purposes via wireless or Infrared.