To be fair, those appear to be pages which list existing packages, which is not what I was commenting on in my OP. However, I would suggest one of the closest equivalents would be nuget[1]. I think that the history of the languages should be mentioned: Ruby 1.0 was released in 1996[2], while the beta of .Net was only released in 2000, and 1.0 only in 2002[3]. Thus one could perhaps say (maybe unfairly, but nevertheless) that Ruby has had more time for OSS projects to develop. I"m not saying that that is the reason for the abundance of Ruby projects on Github, but I don't believe that the relative lack of .Net projects on Github implies that the needs of a .Net developer is not met by the current OSS offerings.
[1]: http://www.nuget.org/
[2]: http://en.wikipedia.org/wiki/Ruby_%28programming_language%29
[3]: http://en.wikipedia.org/wiki/.NET_Framework