It seems like it lacked advertising upon its release, so I'm posting this.
Portal Stories: Mel is a newly released DLC-sized mod for Portal 2. You're playing as a test subject that was participating in an extended sleep experiment back in Aperture's old days.
A series of earthquakes destroyed the facility and woke you up several years later. You get to use a prototype portal gun that has the same functionalities as the modern one. Guided by Cave Johnson, you're trying to escape and get back to the surface. However, something is trying to kill you for some reason. The story contains some twists, and the end will expand Portal's story to a canon level. Chronologically, it's Portal 1.5.
This mod has everything: Custom soundtrack, voice acting, new obstacles (Red Emancipation fields that destroy objects and kill you too for exemple)...
However, like Portal Prelude, it is VERY HARD. I lost patience and had to use noclip on most of the levels when arriving at the modern testing facility.
On top of all of this, it is free, including the soundtrack. You can get it on Steam [here](http://store.steampowered.com/app/317400/ "").