According to a recent post on NeoGAF, there are many new titles due to arrive on the Steam platform in the foreseeable future.
These games include, but are not limited to, Halo: Combat Evolved, Halo 2, Halo 3, Dyad, Fez, The King of Fighters XIII, Cut the Rope, and Super House of Dead Ninjas. Using the Steam search engine provided in the NeoGAF post, you can in fact find some of these games listed under Apps and Subs.
While some of these titles can no longer be found in Steam's database, a few of the posts are as recent as today. For the games you can still find in the database, an App and Sub history can be viewed. One game in particular to take note of is Resident Evil: Revelations, which was announced recently for PC giving more credibility of the notion that these games may be released on Steam.
No comments regarding many of these games have come from Valve, so fans may have a wait on their hands until official announcements are made.