Steam seems to have outed several previously unknown games. Several users captured screenshots of a stats page that includes new names like Killing Floor 2 and Batman: Arkham Origins Blackgate HD, among other console- or region-exclusive games that have not yet been announced for PC.
The forums (via Joystiq) list several games, but the stats page has been changed to remove the names. Along with Killing Floor and Batman, the list includes BattleBlock Theater and Happy Wars, both of which are only on Xbox Live Arcade right now. A "Half Minute Hero Two" is on the list as well, likely a reference to the sequel that's called Hero 30 Second in Japan.
This is strictly a rumor for now, as we haven't heard plans for those various ports or North American releases. Still, as several different Steam users seem to have caught the screens, it's worth keeping an eye out.