Just found out about this:
http://store.steampowered.com/search/?publisher=Open%20Source%20Publishing
Currently lists only one game 
Eh, @asad3ainjalout ?
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Interesting. Thanks.
nice, yah I have that game installeds, its ok, openttd is a bit better though
Apparently, the tag is community-powered and there are other open-source games on Steam.
Looks like a pretty cool game