It seams that Steam has a malfunctioning AI core running amok, because some sort of malevolent Genetic Lifeform and Disk Operating System has gone crazy and prematurely released special Portal 2 material for a couple of games made by Tripwire Interactive; Killing Floor and The Ball.
As of now, you can fight off Killing Floor's horde of undead "Specimens" inside what appears to be an abandoned Aperture Science facility that contains piles of lonely cubes, giant red floor buttons, and a new voice-over that sounds rather like an Aperture Science AI core.
A press release from Tripwire states: "This is a complete, fully-functional level, but was not yet due for release. While we are investigating the source of the leak, everyone who owns Killing Floor is welcome to explore the new level and let us know what you find! We suspect that this may be part of the bedlam that has been going on since Potato Fool's Day."
The aforementioned "Potato" refers to the fact that Killing Floor was released as part of Steam's Potato Sack bundle of games. Another Tripwire Game that was part of the Potato Sack is The Ball, which also has an entirely new Portal-themed level to play through. The Ball and Portal are a natural fit; both are first person shooters/ puzzlers, which give players non-combative "Guns", and task them with shepherding around large geometric shapes, like balls and cubes. In fact, this new level even makes a little reference to the Weighted Companion Cube, by referring to the giant ball that players control as an "Edgeless Artifact Cube".
Both games are on sale on Steam right now, individually, and as part of the Potato Sack.