Earth2Tech has a list of 13 startups working on concentrating photovoltaic (CPV) solar technology. CPV systems, like the one from Australia-based Solar Systems (shown in the photo), work by using mirrors and lenses to focus the energy of the sun onto a photovoltaic cell.

The reason CPV systems are interesting is that the cost of the collector is generally lower than the same area of solar cells. The catch is that the solar cell needs to be cooled (solar cells operate best when cool). Also, a CPV system is less effective on overcast days because the light striking the concentrator is diffused by the clouds.

Stirling Engine
Stirling Energy Systems is developing a similar system, but replacing the photovoltaic cell with a stirling engine. Their first project, in the Mojave Desert in southern California, is planned to generate 850MW of power, and the second (also in southern California) will reach 900MW.