Steam maintains a database of game content. New games are frequently leaked through this database, as games need to be added before they can be released. Recently we saw Brutal Legend appear.
Here's a tool to look at the Steam database:
Today, some truly messed up stuff is showing up*:
Duke Nukem 3D: Atomic Edition (Duke Nukem 3D+) <-- available Mar 20, 2013
Super House of Dead Ninjas <--- available February 18th, 2013
Dyad <-- officially announced by developer Feb 4, 2013
Retro/Grade <-- Officially announced Mar 5, 2013
Angry Birds Space <-- Mar 7, 2013
Fez <-- May 1, 2013
Cut the Rope <-- Mar 22, 2013
La-Mulana <-- Apr 15, 2013
Yogventures! <-- Confirmed
Octodad: Dadliest Catch Beta <-- Confirmed
Resident Evil: Revelations <-- Dated
Shadow Warrior Complete <-- Confirmed
Fast and Furious 6 <-- Released
The Splatters <-- Released as Super Splatters
THE KING OF FIGHTERS XIII <-- Announced for release
Lococycle <-- Feb 14, 2014
Still not announced yet:
LucasArts Test App
THE KING OF FIGHTERS 2002 UNLIMITED MATCH
THE KING OF FIGHTERS '98 ULTIMATE MATCH
Quantum Conundrum 2
Halo: Combat Evolved
Angry Birds Seasons
Normally, you'd be all like "wtf, this is clearly not real", but...
Take Halo 3
The entry links to a Steam Community hub for Halo 3:
Duke 3D's entry even has an icon:
As does Shadow Warrior's:
You can see it right there, with your own eyes. This is a Steam URL with Halo 3 displaying in it.
1. Valve is adding support for Steam Community hubs for non-Steam games
2. Some or all of this stuff is coming out on Steam. We do know that The Splatters is getting a PC port. We do know that Fez is getting a port to new consoles, and that PC is the most likely console. We have seen a few previously GOG-exclusive titles get Steam releases, so it's possible that Duke 3d and Shadow Warrior might fit that pattern. We do know that Twisted Pixel intended PC versions of Ms. Splosion Man, so Lococycle might not be a stretch. We also know from an MS job posting
that they were looking for staffers to evaluate more releases on Steam. We also know that GFWL is basically folded at this point, and that MS has actively worked to make non-GFWL versions of its games available through Steam (see recently the Amazon XBLA PC port pack, all of which activated on Steam).
* Many of the entries predate today, it's just that this particular tool has begun linking the Community Hub data to the database/registry data, and so the information is newly discovered.