It's a power meter. There are a few different kinds but they all have some amount of strain gauges that are strategically placed (rear hub, cranks, spindle, pedal) that measure torque which is converted to watts. The biggest problem is they are expensive! So you've gotta be rather motivated to get one. Which one you get usually comes down to budget, but there are trade-offs based on how it is set up - such as Power Tap is in the wheel, so you always must use that wheel if you want power; however it is easily changed between bikes. SRM is in the crankset so its difficult to move from a bike, but you can use any wheels.
Ombah wrote:I guess you use something like this to get the numbers?
Or do you use bigger things? What should you look for when you want watt mesurement?