Portal Stories: Mel, is a free fan-made Portal 2 mod that takes place between the events of Portal 1 and Portal 2. It does require you to own Portal 2. Portal 2 however, does not need to be installed in order to play Portal Stories: Mel.
It is a result of four years of work by a team of modders (Prism Studios). Portal Stories: Mel (PS: Mel hereafter) has a total of five chapters, custom soundtrack, 350 unique in-game lines along with a custom story. That is not forgetting a bunch of new textures and assets along with of course, a whole new assembly of test chambers to solve and explore. It is definitely like a whole new Portal game to explore and enjoy. With some difficult puzzles. The cinematic cut-scenes are beautifully made, especially that ending cut-scene.
Welcome back to Aperture, old friend.
Fun fact: I downloaded this after falling for their April’s Fool prank thinking it would really be removed from Steam. Only realized it was a joke several minutes later. GJ to me