UnityDecompiled
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Now unnecessary, use the official code instead: https://github.com/Unity-Technologies/UnityCsReference
Unfortunately I had to remove the UnityDecompiled repo due to a request from Unity's legal team.
If you'd like to decompile Unity yourself, find your local UnityEditor.dll and decompile it using either ILSpy or dotPeek - it's easy!
Thanks to the community for all of your contributions over the past three years, it was a great resource while it lasted!
-Matt
UPDATE: Unity has now released an official repo of their C# source code!