I am looking to collaborate on open source simulation efforts.
There is in fact a quantum computing topic tag on github which comes up with a list of a whole bunch of projects.
QISKit is pretty well known; it's connected with IBM's quantum experience as well as the QASM language. It's a pure simulator.
In another direction, there's projects like OpenFermion which is for finding algorithms to simulate different problems in quantum chemistry.
A fabulous little simulator that I prefer over IBM's is Quirk which runs in your browser and is open source.
You can also start your own project like I did, though I would not recommend becoming an open-source contributor to mine because I haven't updated my repository in a while and I'm pretty sure everything there is rather decisively out of date.
If you can provide some more criteria, I can recommend some more specific projects.