*cough*
Well, I know many other members can give you a more detailed rundown on the various CS's, but I'll try my best:
Iron Kingdoms by Privateer Press - My personal favorite out of the 3rd party ones. A steam-industrial world with guns, war, and magitech style elements (steam powered golem-things). Very grim-n-gritty, throws out a lot of the standard D&D tropes (such as the planes of existence), magic is generally much lower in power with healing magic very difficult and often dangerous to perform. Raising from the dead is almost impossible. Very cool setting, even if I'm not a big fan of how some of the rules are handled.
Scarred Lands by Sword and Sorcery Studios - A very popular setting that has just gone under. Has a lot of product support for it (too much, some may claim). The basic premise is that the titans battled the gods and lost. In the process, the world was, well, scarred and the surviving races struggle to eke out an existence in the very changed world. Grimmer than standard D&D, but has most of the standard tropes.
Midnight by Fantasy Flight Games - Think of Midnight this way. Imagine if Sauron had won the war in Lord of the Rings and you pretty much have Midnight. Much lower in magical power than standard D&D, less heroic (IMO) and more of a struggle to survive in a dangerous world, and, of course, very dark. Very cool setting, even if I don't actually own the book. I'm cheap
Diamond Throne by Malhavoc - Truth be known, all I know of this setting is that it's the setting behind Monte Cook's
Arcana Unearthed. Pretty much rebuilds a lot of the rules surrounding D&D while still managing to stay d20.
Oathbound - I know exactly jack-squat about it.