Comment by neop1x

There is also Unigine [1] but that has less community around it than Unreal. I am mentioning it because I think it should be known more. It was started by a single russian dev who was initially just writing online tutorials on various OpenGL and physics stuff (frustum.org). Then he decided to make a commercial engine and started a company. Of course their engine evolved a lot since and they have a team of devs now and are no longer based in Russia (and the original dev is no longer in their team I think?). I am impressed by what they have achieved (it must have been super-hard especially in the beginning) and that they continue developing it and the graphical output is quite competitive to the major engines. They are also known for various graphical benchmarks.

[1] https://unigine.com/