Originally Posted by
ukmtk
Wouldn't a GPS based cycling computer do something similar?
No - the GPS can measure how fast you ride and how much you climb, but it can't tell how aerodynamic you are on the bike.
Powertap and SRM power meters work be measuring how much force you put into the pedals. Powertap measures it in the rear hub, and SRM measures it in the crank. Powertap is $$, SRM is $$$$.
Polar makes a power meter that measures chain tension based on how much it sags. It's had mixed reviews.
iBike makes one that estimates power by measuring how much air is running over the bikes. It's not measuring power in the same way the others are, and you should look around for reviews if you're thinking of buying.