Montage was a window system I wrote as part of my undergraduate senior thesis. I was trying to explore ideas for making window systems easier to program. This lead to a concurrent process system for input and a graphics model based around compositing; I still believe that the ideas are useful and easier to work with than current methods for writing window systems. Looking back from six years later, even more of the work seems derivative of Rob Pike's ideas than it had at the time I did it.
The system was described in Montage: Breaking Windows into Small Pieces, a paper I presented at the Summer 1990 Usenix conference.
I would like to make the source code available, but I have not been able to find it. When I do, it will be available from here.