Has anybody else been playing this? It is a free mod for Portal 2 that you can download through Steam - you just have to have Portal 2 in your library. Without saying too much, it is a full story with all new puzzles, voice work, etc. I actually just finished it, and I found it to be excellent. I think referring to it as a mod is perhaps selling it a little short - I thought it was impressive enough that it could very easily be called Portal 3.
If you enjoyed the Portal games at all, this is well worth checking out. Fair warning though, it is considerably more difficult than Portal 1 and 2.