Power Meters

Power meters measure the amount of work (in watts) a cyclist is performing.  They are useful for training, much like a heart rate monitor.

Popular brands include:

• Power Tap  Can be extremely finicky in the rain, but because of its reasonable price and being good in other ways, quite popular.
• SRM  The best powermeter in the market, but oh so expensive! German engineering at its finest.
• Ergomo  Works well, but the cycling computer is so big it looks like a depth finder on your bike, and it lacks on-the-bike functionality (or at least used to) like bike splits. However it seems quite accurate and the design concept seems sound. You can use different wheels with it.
• Manufacturer site: http://www.ergomo-usa.com/
• Polar Power Meter  Criticized by some as being quite flakey. Interfaces with the Polar S710  heart rate monitor. By the time you buy both it's not that much cheaper than a Power Tap.

Very detailed power meter review at biketechreview.com