The above trailer is the work of Portal 2 machinima-maker 'Harry101UK'. The mod powering it is Portal Stories: Mel. It's a new story, based in the Portal 2 universe, about new protagonist Mel and the new personality core that she's travelling with.
The mod boasts custom textures and animations, and over 20 new maps. It's also been Greenlit—meaning it will be made available directly through Steam. And, unlike last year's Aperture Tag, it will be entirely free.
For more, head over to the mod's Steam page. Portal Stories: Mel is due out early-2015.