There seem to be a spate of "bare-bones" carbines being offered with no furniture. First was the Colt OEM line, and now Aero Precision is offering a similarly stripped carbine. If you own an AR-15 already, chances are pretty good you've picked up a set of furniture (or two, or three, or...) to outfit one. Even if you are simply looking to upgrade or add utility, there are plenty of options.
Blackhawk's AR-15 Carbine Quad Rail Fore-end offers ambidextrous QD attachment points, simple installation and 15-slot rails at 12-, 3-, 6- and 9-o'clock positions. Simply unscrew the two halves, install on the carbine like any other two-piece fore-end and replace the screws for a solid, sturdy attachment system.
At the end of this past summer, I found myself out in Montana as a guest of Blackhawk for a product rollout event. The folks at Palmetto State Armory outfitted our group with PA-15 carbines, and one of the upgrades we made to the rifle was the addition of the Blackhawk Quad Rail fore-end. It allowed attachment of a vertical fore-grip and rail-mounted lights, and proved to be extremely solid and durable as we ran a simplified 3-gun course on a nearby range.