Hmm... HMM... Steam users are reporting that, for a brief period, visiting the store's stats page sent them instead to the Shadow Steam: a secretive and underground society where developers engage in spectacularly tense battles of "Petroglyph Games Developer Test App". The list appeared to forego the public applications of the Steam database, instead reporting those marked as private. Kind of like the exact opposite of what it should be doing. Of course, things are back to normal now, making it difficult to prove the veracity of the list. If the community's claims are true, though, it would suggest the existence of unannounced games like Killing Floor 2, Batman: Arkham Origins Blackgate HD, Half Minute Hero 2, and Battleblock Theater.
Plenty of intriguing names crop up on the list, including a Steam release of Disciples: Reincarnation, known projects like Next Car Game, dev builds of existing games, and the intriguing presence of "Super Secret Project 1". Of those that are new, there are ports of the handheld game Arkham Origins Blackgate, and Behemoth's XBLA game Battleblock Theater, as well as the yet to be announced Killing Floor 2.
It could all, of course, be complete tosh, and, even if it isn't, there's no guarantee that the listed games will ever be released. With that disclaimer in mind, you can view the full list over at the Steam forum.