Portal Stories: Mel is a community made, free modification for Portal 2 based in the Portal universe. It tells the story of Mel, who meets a new personality core and faces an undiscovered threat to the Aperture facility.
I downloaded this when it came out but have only just gotten around to playing it. It obviously lacks the polish of a multi-million dollar Valve game, but it is still exceptionally good. I particularly liked the Half-Life homage tram ride at the start.
Portal Stories: Mel is a standalone mod that does not require Portal 2 to be installed to play, though you do need to own Portal 2. The game has 22 levels, with an estimated gameplay time of ~6-10 hours.
See also: The Cabal: Valve’s design process for creating Half-Life and other posts on this blog tagged ‘games’.