I read this on Lifehacker this morning, and it looks to be a new (free) mod which works a lot like a sequel to Portal 2.  There are new puzzles, including a new "time travel" mechansim in addition to the normal portal mechanism.  I don't know any more than that, but will be downloading this when I get home tonight.

https://lifehacker.com/download-this-free-mod-to-get-new-mind-bending-portal-2-1846723514