I've kept almost every bit of scrap PLA I've had from printing. You can melt it into silicon molds, grind it up and feed it into a pellet extruder, pass it on to those who recycle it into filament (like https://erikaprintsshop.wordpress.com/), or for the hard-core DIYers, create your own filament recycling operation. (Though more approachable commercial options are starting to come to market (https://crowdfunding.creality.com/)