It's been over a year since Portal 2's release, but Valve's still dropping new content to keep fans going. Today, Valve just launched the Perpetual Testing Initiative free DLC that's actually the game's map editor.
The DLC includes a "simplified puzzle maker", which provides a drag-and-drop interface for users to generate levels. Perpetual Testing Initiative DLC will also allow players to scroll through player-built levels and vote on them through a "thumbs up/thumbs down" system.
You can look through the Steam Workshop to check out the user-generated levels. Even though the DLC just launched, there's already 5,963 entries at the time of this writing. Whether you're an avid Portal 2 player or one that's fallen off the map, this'll be your motivation to never stop playing… exactly as Valve planned.