Mountain Lion - scale down the window shadows
The program I use to do this on OS X 10.8.4 is ShadowSweeper.

http://download.cnet.com/ShadowSweeper/3000-2072_4-75966596.html

This one looks like it might also work but I haven't tried it myself.

https://github.com/puffnfresh/toggle-osx-shadows

Others have suggested the programs ShadowKiller and Nocturne. Neither worked for me.