Both the ZM Weapons (Yankee Hill Machine) and Olympic Arms OA93 systems move the recoil system from the receiver extension (buffer tube) to the upper receiver assembly, allowing for a folding stock.  Note that neither of these are gas piston systems, as is often posted or assumed.  Both use the standard direct-gas system.

As others have noted, a folding stock will NOT work with a standard AR setup.  Folding stocks are useless for the most part anyway, whereas adjustable-length stocks are very important to deal with differing shooting styles and equipment (heavy clothing, backpack straps, body armor, etc.).  They also allow you to teach smaller- (or larger-) statured people how to shoot using your rifle, with just a simple adjustment of the stock length.

-Troy