I just registered at CryEngine because of your comment. Unfortunately, I can't find out how I can get the source after registering for an account. There's only a link to a 'launcher', but it's a .exe and doesn't seem to run on Linux. Googling lands me in their manual that does have a page on compiling for linux, but nowhere does it say how to get the source.

Maybe I'm not a typical game developer, as I don't like to do my actual developing on Windows, but in UE4 I've always felt as a well-loved second class citizen.