The AP had a story a few days ago on Ooms Avenhorn, a Dutch company that has found a way to use some of the heat that road naturally suck up to heat office buildings. The system (first imagined ten years ago) certainly seems efficient: one test patch - made up of 200 yards of road and a small parking lot - generated enough heat for 70 apartments in a four-story building. As the AP writes, this happened "under normally cloudy Dutch skies, with only a few days a year of truly sweltering temperatures."
How does it work? I'm glad you asked. Water is run through pipes under the asphalt and heated from the warmth of the road. This heated water is then pumped underground and stays warmish at 68 degrees F. The water can be sent to nearby houses (where it must be heated a bit more) and, during winter months, this water can be pulled up to prevent ice from building up on the road. The pump can also call on the water to cool the buildings on hot summer days. The problem? The pipes and the rest of the system double the cost of building the road.
Check out the AP story for details, and you can also download a PDF from Ooms Avenhorn describing the technology.