Atlona (Dual Link) DVI to Mini DisplayPort Converter for Apple 27-inch LED and 27-inch iMac 

The Atlona AT-DP400 is a DVI to Mini DisplayPort converter designed to convert every Mac or PC computer's with a dual link DVI output to a digital signal over a Mini DisplayPort connector. The AT-DP400 is specifically designed to work with iMac 27" which can support resolutions up to 2560x1440. Users can use the connected monitor as their main display or extend their desktop using it in addition to their laptop's screen.