While more and more closed-source games -- ranging from indie titles to AAA games -- are heading to Steam ahead of SteamOS and Steam Machines, there still isn't too many open-source games on Valve's digital distribution platform. Here's some information why one of the more popular cross-platform open-source first person shooters isn't yet to be found on Steam.
Posted today on the Xonotic blog is why the first person shooter isn't yet on Steam. Xonotic is a very popular open-source game and being forked from Nexuiz is one of the most visually impressive open-source games. Xonotic isn't your AAA title, but its DarkPlaces-enhanced engine and creative visuals and in-game assets are much better than what you normally see from open-source games. Xonotic is also one of my favorite open-source games to use for GPU/driver benchmarking due to its visual intensity.
Xonotic developers aren't in opposition to Steam, but there's some "problems" they must solve first:
rest here