Portal: Revolution is a huge, eight-hour mod that bridges the gap between Portal 1 and 2
40 fully-voiced test chambers
Portal: Revolution is an eight hour fan-made Portal 2 prequel, with 40 new test chambers, voiced characters and new puzzle mechanics. It looks seriously impressive and it's out now, with a trailer below.
Voiced characters and dialogue always seem to be one area where mods fall down, much more than level or puzzle design, but the above trailer gives me hope Revolution overcomes that particular obstacle.
Revolution takes place in between Portal 1 and 2. GladOS has been destroyed, sending the Aperture Science Labs into chaos, and you - an unnamed test subject - and a personality sphere called Stirling work to find a "powerful device capable of restoring Aperture to its former glory."
Although set before Portal 2, the game makes use of Portal 2's mechanics, including all its paint gels to make surfaces slidey or springy. It also adds some new ideas of its own, including a new laser cube variant and a "gel cleansing field" - which presumably strips gels from objects you're holding when they pass through, which I think can be seen in the above trailer. It also makes use of Pneumatic Diversity Vents, which were transport tubes initially conceptualised by Valve but cut from Portal 2 before release.
Although set before Portal 2, and making use of the ideas of Portal 2, the difficulty "starts where Portal 2 stops", according to the Portal: Revolution Steam page. Although it also says to "fear not", because they say the puzzles have been thoroughly playtested to make sure that new ideas and advanced tricks are taught to the player via the levels. "Although very few chambers require portals to be fired mid-air, no puzzles require advanced tricky movement."
Portal: Revolution had originally inteded to release on January 5th, but experienced a brief one-day delay while waiting on Valve to complete the review process all Steam games go through.
I loved Portal 2 upon release, particularly its co-op mode - which remains, arguably, the best co-op game - but I haven't returned to it since. Revolution is a very tempting excuse to start a replay.