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Rigetti Computing has a Github repository with a reference implementation for their quantum virtual machine and other stuff: https://github.com/rigetticomputing

There is also Quantum computer playground by Google: http://www.quantumplayground.net/

Here's list of various QC simulators: https://www.quantiki.org/wiki/list-qc-simulators

Microsoft's Language Integrated Quantum Operations Simulator is available on Github http://stationq.github.io/Liquid/

That is not open source. Note that the meat of it is in the form of compiled binaries in the /bin directory: https://github.com/StationQ/Liquid/tree/master/bin

In this case, GitHub isn’t being used so much as an open source collaboration tool, but rather a file host.

There's Quipper - a language for describing quantum algorithms:

http://www.mathstat.dal.ca/~selinger/quipper/