Fo power data freaks we have SRM (perhaps the ultimate bike accessory,
using force transducer-equipped cranks), PowerTap (using a
transducer-equipped re hub), Polar (least invasive, using optical sensing
of chain deflection) and now... Ergomo. It's optical, but it's inside the
BB! Check it out at http://www.cbike.com/ergomo_powertraining.htm
OK, you can also use 'manual' calculations based on effort over time, like
the HAC4 does. That's the cheapest, lightest and probably least accurate
way to do it.