X Pseudo Root
I wrote this extension a long time ago. Long enough ago that I can't find the code anymore, although it may be sitting on a SparcStation disk in my closet, that machine no longer boots for me to easily check. I presented a talk about it in 1995, and there's a paper and various notes from that era linked from my talks page.
In any case, the idea has come back around as something that might make encapsulating full-screen games work better. By burying the game inside a 'pseudo root', we can things like the actual screen resolution and multi monitors.
This extension works by dynamically creating additional X listening sockets; connect to one of the new sockets and you get a different root window, one owned by an external 'pseudo root manager' client. Within that nested window, you can run a separate window manager.