Update since original story broke: A Microsoft spokesperson spoke to Eurogamer stating:
“We currently do not have plans to release any ‘Halo’ titles on Steam.”
A large list of games which are allegedly due to be released on Valve’s digital platform, in some form, in the future has been leaked via the Steam Apps Database. The Steam Apps Database of which is a website dedicated to displaying Steam registry data in a way which is easy for us common-folk to understand.
Notable titles featured in the list include:
Would Steam versions of these titles interest you?
[Source: Steam Apps Database via Eurogamer]