Filabot, "The Personal Filament Maker" is an ongoing open-source hardware kit project that aims to perfect a plastic grinder/melter that you can use to turn scrap plastic (including failed 3D printouts) into filament that can be fed into 3D printers. It's a promising step towards the blunderbussification of 3D printers, turning them into devices that can use any random junk as ammo for useful work:

The Filabot Reclaimer, is our flagship system, that allows for the already innovative 3D printing movement, to become more self sufficient, experiment with new materials, and recycle bad prints.

The Filabot Reclaimer includes the grinding, extruding, and spooling systems. The Grinder will tear up bottles and can handle up to a good 3in by 3in chunk of plastic. Material from the grinder can either be stockpiled or fed directly into the extruder. From there the extruder will melt and pressurize the molten plastic to push it thought the interchangeable dies. There are two dies included with the Filabot Reclaimer, a 3mm and 1.75mm, depending on the filament size needed. The spool system will automatically roll the filament onto a spool after cooling and sizing.