Watch for announcements in the coming weeks about all the AAA titles coming natively to SteamOS in 2014.
Valve have a big problem with threes.
L4D, L4D 2
Portal, Portal 2
Dota, Dota 2
we even have
HL2:E1 and HL2:E2
Valve is standing on number two in nearly all of their game series.
Source 2 is unveiling number three (number two is steambox),
Valve games generation three will be based on Source 2 and all
of them will feature launch day steam play (win+mac+lin)