The DTAC Mongoose is a minimalist stock system for the Mossberg 500 or Maverick series. It features a quick detachable stock so you can switch to a pistol grip shotgun without the need for tools. The most interesting feature is the unique dual chamber, pneumatic/mechanical recoil reduction system. This offers dramatic felt recoil reduction (about 75% with the heaviest loads), which means comfortable shooting and quicker follow up shots. A tad bit pricey though at $205 MSRP.
If you are interested, check out DTAC for more information.