Back to RPi Expansion Boards.
The Gertboard is one of the expansion boards, sold for some time in a kit form. It was nicknamed after Gert van Loo, its developer and one of the principal Hardware Engineers for Raspberry Pi, and was announced on 17th December 2011: 
Geertboard Availability Announcement includes links to assembly and user manuals.
Additional Details can be found on the Raspberry Pi Forum here.
As of 2012-11-01, it is not possible to order Gertboard. In Element14 words: "Sorry to disappoint, we have decided to remove the Gertboard from sale. Don’t worry we are planning on producing a fully assembled board but this will not be available for another few weeks."