The Light-Net.

Some bright scientists in Boston (with a little help from the government) are turning energy-efficient lights into flickering wireless repeaters:

The technology will be able to communicate data with visible light at up to 10Mbps, and can be adapted to existing power lines.

The Register:
The data would be transmitted by the same LEDs which provided white-light illumination, flickering like tremendously fast signal lamps. The data flicker would be imperceptible to humans.

As visible light doesn’t penetrate walls or travel round corners, the developers say that the system would be more secure than present-day WiFi, and would avoid the problem of multiple radio networks competing for bandwidth. Furthermore, the much higher frequency of visible light compared to radio waves would mean there was much more wireless bandwidth to use.