Last year Valve sponsored DebConf 16 and this year they are back to their golden sponsor tier for backing DebConf 17 in Montreal, Canada.
Valve has sponsored DebConfs and offered various perks to Debian developers due to their Linux gaming interests and in large part because their SteamOS operating system is based on Debian.
The first DebConf 17 platinum sponsor is Savoir-faire Linux. Joining Valve at the gold sponsorship level is Collabora. There are also lower sponsor tiers including Ubuntu, Microsoft, and The Linux Foundation, among others. As far as Microsoft's involvement, they do sponsor various Linux events going back years.
The current DebConf 17 sponsors can be found via bits.debian.org.