My (less-than-half-baked) master's thesis has a screenshot of Xnest, which I was using to demonstrate some ridiculously primitive precursor to kernel namespaces, IIRC
One of my favorite bits of software is Xpra [0], "screen for X". You'd run it and it would start another X server (start apps in it with `DISPLAY=:1 xterm` or whatever), and you would "attach" it to your running X server with `xpra attach`.
You can attach to e.g. `ssh://hostname/:1`, so I ran a firefox instance on a homelab server and attached to it from my laptop and my desktop to not have to bother keeping bookmarks, tabs etc in sync.
I recall, around 2004, using it on Windows, under Cygwin, to run cygwin-compiled X apps on Windows.
I'm not sure how that worked at all, to be honest. I hope my memory is not playing tricks on me again.