I was making a list of a few I knew, but it would be better to just check wikipedia: https://en.wikipedia.org/wiki/Comparison_of_operating_system...
Those are more serious kernels I think. There are also a ton of research-related kernels. I guess you can find a few for some programming languages.
Here is information and links about some unusual operating systems, including several discussed above.
https://github.com/jon-jacky/Piety/blob/master/doc/precursor...
I don't know how an OS would qualify to get on the list other than not DOS, Windows, OSX (or any of the Apple OSs), Linux (especially not 'ubuntu with a different DE'), or the BSDs (i guess there are exceptions if they're on a toaster).