Strava

I love using Strava but I think the power thing really depends on your area.  If you live in say, Kansas, where the wind is solid one direction and the rolling resistance (i.e. smooth roads) then it may not be too bad.  However, if you live at the base of a mountain range were wind hits in mulitple directions I wouldn't trust it too much. 

Regardless its still fun to look at the numbers.

You have to be accurate on your self+bike weight, and it does better on long loops.  If it's a short segment in one direction, it can be way off.

You can fine-tune your weight settings - if you borrow a friend's Powertap wheel for a day and do the long ride thing.  If the average power comes out higher than your typical Strava result, raise your bike weight a pound or two.

I did that with my commuter (normally don't have power on it).  Strava now usually gets within about 10% of my typical power.  Don't trust their estimate for any kind of training though - it's not that reliable.