Boot supported iMacs straight to Target Display Mode, without booting Mac OS X first.
Works on at least my 27 inch 2010 iMac, but should work on all 2009 and 2010 21.5", 24" and 27" iMacs. It will NOT work on iMacs released in 2011 or later with Thunderbolt ports (yet) and will not work on any iMacs Apple did not officially support for Target Display Mode (sorry!)
Sound might work but is untested. Brightness control also does not work, and CPU core 0 is pinned at 100% at all times currently (resulting in high power consumption and the fan spinning up).
Releases can be found here. The source code can be found here.