I use these on my 870.
A UTG forearm with three 6" rails 3 covers per rail should you choose not to use sections of them. It is bigger than the stock forearm but does give a solid grip. It does come with that red laser sight, which I have not used. It is also shorter than the stock forearm which allows my next item...receiver rail with side saddle shell holder.
I could not find a link to the 870 model, but here is the Mossy version at Amazon.
http://www.amazon.com/Tactical-TriRail-UTG-Mossberg-Shotguns/dp/B001G8FWMQ/ref=sr_1_7?ie=UTF8&qid=1315935335&sr=8-7A Mesa Tactical Sureshell Shotshell Carrier Saddle Rail. It has a 5" rail positioned above the receiver. The installation kit has two tapped replacement pins for the 870 stock receiver pins. The Saddle Rail is bolted into the tapped pins, two bolts on each side. the shotshell carrier holds 6 rounds. It can be found here
http://www.mesatactical.com/index.php?id=56Last for rails, I have a Nordic Components 2 shot mag extension and use their barrel/extension clamp which has a rail.
I cannot find the 870 product on their home page, but the part number is NC-EXT-RM-12-02. The 02 is the 2 shot extension. I was going to mount the laser from above on the clamp rail if the activation cable would stretch enough to avoid it moving with the slide....but haven't gotten to that.
Here is the link for the extension tube, again for a Mossy, and then the clamp
http://www.cactustactical.com/osc/advanced_search_result.php?keywords=nc-ext-mb-12&search=Searchhttp://www.cactustactical.com/osc/nordic-components-gauge-shotgun-extension-tube-clamp-with-rail-p-2112.htmlNo rail here, but I use the Knoxx Specops Adjustable Stock with a 5 shot sidesaddle shell holder, for a total of 9 rounds carried external and 8 rounds in the tube and chamber.
http://www.blackhawk.com/product/SpecOps-Adjustable-Shotgun-Stock,1158,165.htmhttp://www.blackhawk.com/product/PowerPak-Modular-Cheek-Piece,212,167.htmI like the setup a lot, but then I found a new Saiga 12 for under $400.00 and have started working on it.