Well, there was the factory Remington 870 folding stock, which had a bad tendency to cut the cheek of any shooter trying to maintain a decent cheek weld while shooting. These are hard to find but if you look hard enough you can find one (or if you are LE you can get one direct from Big Green).
Then there's the aftermarket offerings by Choate Machine & Tool, available from Bushmaster Firearms (http://www.bushmaster.com).
Beyond that, I don't know if there are any others.
Justin