Derailer/hanger for Cannondale Jekyll
http://wheelsmfg.com/media/catalog/product/cache/1/image/299x299/06866988214b6a9b717ef7211319fd38/d/r/dropout-16.300px.jpg
I think it may be this one (Wheels Manufacturing #16). Is there a broken bolt in the hole just above the rear axle? You will need to find a way to get that out to be able to mount the hanger properly.

For a derailleur, it would be nice to match it with the front, so look for a 9-speed Shimano XT. Try to avoid low-normal (Shimano calls it rapid-rise) derailleurs. Any Shimano 9-speed mountain bike derailleur will work (Deore, LX, XT, XTR). I wouldn't go below the Deore level to keep similar quality level for all components.