Parabolic dishes used for satellite reception have been around for a long time and can be adapted to solar use. These feature very high concentration factors, but also require very accurate dual axis controllers for best results. They also require large sturdy stands and aren't suitable for many locations.
Infinia Corp has a 15 foot diameter parabolic dish that produces 3.2kw utilizing a Stirling engine generator system, but they seem to be oriented toward utility customers, probably because of cost which they don't list on their website.
Stirling Energy Systems had me excited for a while with their 25kw dish system, but after an embarrasing round of project cancelations where they lost out to PV, they recently anounced bankruptcy.
I'm hoping to come up with a micro tracker system that can make use of the small satellite dishes which are popular right now. I covered one with some reflective mylar, and it instantaneously ignited a piece of paper I held up to test. Either some high temp PV cells or a micro heat receiver would make a nice micro CSP package.