It appears to be a VESA DMS-59 connector.
From Wikipedia:
"DMS-59 is a 59-pin electrical connector generally used for computer video cards. It provides two DVI or VGA outputs on a single connector. An adapter cable is needed for conversion from DMS-59 to DVI or VGA, and different types of adapter cables exist. The connector is four pins high and 15 pins wide, with a single pin missing from the top row, in a D-shaped shell, with thumbscrews.
The application for DMS-59 is to support a higher density of video displays, in a similar amount of connector space to a DVI connector. DMS-59 supports two DVI Single Link digital channels or two VGA analog channels from a single connector. The compact size lets a half-height card support two high resolution displays, and a full-height card (with two DMS-59 connectors) up to four high resolution displays"
Edit: Dammit, beat by 1 minute