I recently purchased a Cycle Ops PowerCal from OCW. This was not a stock item, but they had it in my hands in couple of days. The retail cost is $99, but saved some dollars with our club discount.
The system uses algorithms to calculate the correlation between heart rate and power in real world conditions. It's advertised as easy to use and set up and ANT+ compatible. True to their advertisement I paired the PowerCal with my Garmin Edge 500 in about 10 minutes, and off I went for a ride.
This shouldn't be confused with a Power Tap or SRAM power meters which will cost you well over $1000! It's not as accurate as a Power Tap, but since the data seems to be repeatable I'm not worried as to how they compare to someone else, but how they are relative to one training secession to secession. As an example I can compare last week's intervals secession to the intervals I'll do tonight.
As an added bonus you are not limited to using a Power Tap wheel or a specific bike. I mount my computer on which ever bike I'm riding that day and off I go.