See the article and pictures
here.

At the COP 15 Climate Change Conference in Copenhagen, M.I.T.
students introduced the technologically advanced Copenhagen Wheel. In addition
to including various sensors and Bluetooth capability, the tire stores kinetic
energy from braking for a later burst of speed.

The makers of the Copenhagen Wheel from M.I.T.'s SENSEable City Laboratory claim
that the new features mark the advent of "Biking 2.0," a new era based on
smarter bikes and easier rides. The wheel is certainly a step in that direction;
it includes sensors for detecting distance, speed, direction, all of which are
beamed via Bluetooth to the rider's iPhone. The wheel also includes a built-in
lock that sends the rider a text if tampered with.

But the most notable feature of the Copenhagen Wheel is its KERS or Kinetic
Energy Recovery System, a mechanism by which energy from braking is stored up
for later use, giving the rider a boost when going up a hill or speeding through
traffic. Some bicycle purists have already dismissed the wheel as a novelty
while others suggest that M.I.T. has succeeded in reinventing the wheel.