As we retweeted last night from one of the Unity developers (too late to do an article when I saw it), they've now opened up the UnityEngine and UnityEditor source code for referencing.
The key thing to note here, is that Unity is not making steps towards their engine and editor being open source, they do say it's specifically for developers to reference and to understand what it's doing behind the scenes. Something the Unity developers hope might help developers.
It's interesting to see them do this, hopefully they will reconsider their stance on actually open sourcing it in future. They make it clear they're not against it, but they don't think they would be able to remain in business if they did (source):
We are not releasing Unity as open source. Not even a little bit. (Sorry.) It’s not that we don’t like open source. We’d open source all of Unity today if we thought we could get away with it and still be in business tomorrow, and we do have a growing number of open source projects. But the main engine will remain proprietary for the foreseeable future, and the C# reference source code is released under a license which only permits you to read the code, not modify it. Please consult the full license text for details before you get carried away.
You can find it here on GitHub.
Ps. If you do need a fully open game engine, there's always Godot.