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:
http://cdr.xpaw.ru/

Today, some truly messed up stuff is showing up:
La-Mulana
The Splatters
Octodad: Dadliest Catch Beta
Duke Nukem 3D: Atomic Edition (Duke Nukem 3D+)
Super House of Dead Ninjas
Fez
LucasArts Test App
Fast and Furious 6
Lococycle
Yogventures!
Hardware
Hardware (Demo)
Dyad
Cut the Rope
Shadow Warrior Complete
Hardware (VIP)
Hardware (Internal)
THE KING OF FIGHTERS XIII
Retro/Grade
Resident Evil: Revelations
THE KING OF FIGHTERS 2002 UNLIMITED MATCH
THE KING OF FIGHTERS '98 ULTIMATE MATCH
Quantum Conundrum 2
Halo 3
Halo 2
Halo: Combat Evolved
The Witness
Angry Birds Space
Angry Birds Seasons
Second Life