If you have been following alternative operating systems for a while, there is a good change that you have herd of Hurd. It has been in development since before Linux, and yet it is still not stable. This kernel with a super cool logo is super powerful. I am going to tell you where you can get it, why you might want to check it out, and some of it’s drawbacks.
You should check 0 A.D. more currently.
One thing is that devs consider it Alpha, other is being not playable at all.