Holy mother of mod, Portal Stories: Mel is awesome. Developed over the course of four years, this fan-made mod for Portal 2 is a standalone game of such quality that it could almost be mistaken for a true sequel to the much-loved series.
I've put my thoughts into the video below alongside some gameplay, but to be honest, if you've got a PC and a copy of Portal 2 you should probably just skip the watching and get straight to the downloading. It's an experience on par with anything Valve has created and you don't have to pay a penny. Bit of a no-brainer, really.
...Although, actually, if you could just maybe load the video and leave it running while the game is downloading, that would be great. Every view counts, and all that. Ta!