I have an "old" PowerTap SL+, which is pretty great. It's pretty impressive how prices are coming down as demand for power meters is increasing a lot from just a few years ago (competition anyone?), making them affordable for the enthusiast cyclist.

Now they just need to figure out how teach people to interpret the data better because humming along at 250W and realizing a pro would be doing the same thing at 350W is futile and doesn't add any more value or information than riding without a computer OR a powermeter.