Dark Sun
I'd write something about Dark Sun, but it's already been said. This is the Dark Sun reference I use in addition to the Campaign Setting for most of the flavor of the setting we're playing. This can be a tremendous help to anyone looking to develop more of their backstory (hint hint, nudge nudge).
http://community.wizards.com/alphastream1/blog/2010/05/17/warming_up_for...