Portal Stories: Mel offers a new adventure, new equipment, and new missions for Portal 2 fans who want to get a new adventure out of a game they've probably owned on Steam for some time now.
If I remember correctly, Valve gave the original Portal away for free but you can still get a license for Portal 2 relatively cheap. Once you do, be sure to vote for Portal Stories: Mel on Steam Greenlight in the hopes that the developers will finish working on it by 2015.
Players take on the role of Mel, a survivor of Aperture's lab testing in over 20 maps. The mod even includes a "prototype" portal gun. For more information, visit the game's Steam page by clicking the link above.