This article by MIT's Technology Review describes a novel technology by Soliant Energy that concentrates sunlight using mirrors and lenses onto a small area and reduce the amount of expensive photovoltaic material needed. Suitable for rooftop installations, this product, dubbed the "heliotube," is a minature version of CSP (otherwise known as concentrated solar power), which are more typically the domain of large scale solar farms built by the likes of WorldWater & Solar Technologies Corp. (see article on CSP).
According to the the Technology Review article:
Soliant has designed a solar concentrator that tracks the sun throughout
the day but is lighter and not pole-mounted. The system fits in a rectangular frame and is mounted to the roof with the same hardware that's used for conventional flat solar panels. Yet the devices will likely cost half as much as a conventional
solar panel... A second-generation design, which concentrates light more and uses better photovoltaics, could cost a quarter as much. He says that a more advanced design should be ready by 2010.