Valve built SteamOS on Debian, and the developer thought that it could do a very nice gesture for all the people who are contributing to Debian.SteamOS couldn't have made the impact that it did without the Debian distro, and Valve decided that it could give something back to the community.
“We've been working with Valve on SteamOS, which is based on Debian. Valve are keen to contribute back to the community, and I'm discussing a couple of ways that they may be able to do that. Immediately though, they've offered a free subscription to any Debian Developer which provides access to all past and future Valve produced games,” said Neil McGovern of Collabora, the company that helps Valve interact with the open source world.
It's interesting to note two things. One, Debian developers will get access not only to all the Valve developed games so far, but also to all the Valve games that will be released.
Two, Valve can apparently issue this sort of pass that immediately grants access to all the games from a publisher on Steam. Where can we buy that?