In brief: Ten years after its release, most people will have mastered Portal 2's puzzles. If you count yourself among that group, check out this new mod not only adds more test chambers, but also introduces a whole new type of portal, one that allows time travel.
With the Portal Reloaded mod, a green portal joins the usual blue and orange ones. It's quite different, though, as it sends you through time---20 years into the future, to be precise. The explainer video shows how moving an object in the present will affect the future version's position, but not vice versa.
Additionally, while you cannot take a cube from the present into the future, players can take a future cube to the past, allowing two versions of the same object in one timeline. The caveat here is that the cube from the present cannot be moved without affected the future version.
Finally, a portal placed on a surface will appear in the same spot in the future, and future portals can be moved around without affecting those in the present. Clear? Good.
In summary, this all sounds like it will be pretty challenging---the mod is aimed at "Portal veterans"---but it's certainly a clever way of adding an extra element to the game. There are a full 25 new puzzles to test your thinking, and it includes more than 100 custom voice lines along with an original soundtrack.
If you own Portal 2, you can download Portal Reloaded from Steam, where it already has an Overwhelmingly Positive rating.