A small team has been working on an extensive mod to Portal 2, titled Portal Stories: Mel, putting you in the shoes of a character named Mel and with a brand new (older model) yellow portal gun. And the work they've done is so huge, it's pretty much an entirely new entry in the Portal series (albeit without GLaDOS).

Per the developer, here's what you can expect:

Portal Stories: Mel is an upcoming free to play mod based in the Portal universe. It tells an entirely new story about a protagonist called Mel who travels with a new personality core and faces a new, still-unknown threat. "What happened to me?" 

Main Features:
- 20+ Puzzle and exploration filled maps 
- Custom Story and Dialogue 
- Custom Textures, Models and Animations 
- 70s Prototype Portal Gun 
- Free to Play (you must own Portal 2)


Here's the mod's Steam page, where you'll eventually be able to get the full version for free (assuming you have Portal 2, and c'mon, you have Portal 2).