There are a few different kinds of powermeters on the market right now.
iBike - estimates power from various measurements, not entirely accurate.
Polar - uses chain vibration, clunky to set up and also has accuracy issues according to some.
Powertap - hub based, so you build up a wheel and are stuck with it, though transfering bike to bike is usally pretty simple (especially if wireless). http://www.competitivecyclist.com/ro...905.351.0.html
- this may be the cheapest, reliable power meter on the market right now.
SRM - Crank based power meter, very accurate, very expensive.
Quarq CinQo - Crank based like the SRM, but about half the price.
FWIW - I currently own a powertap and have been happy with it for the most part, but hate having to choose between having power or running race wheels. A Quarq CinQo is currently being shipped to me. I have used a CinQo in the past and have been waiting to get my own for some time now, IMO they are the best of the bunch.