A window manager for the X Window System, written in 1994-5 by David Hogan. 9wm emulates the Plan 9 window manager. It's a very simple, straightforward system, but the great thing about it is that the code (in C) is elegant and comprehensible. Earlier window managers such as twm, or even the near-trivial uwm, were surprisingly difficult to comprehend in source code. We probably have 9wm to thank (or blame) for an explosion of "minimal" window managers, as people realised these things weren't impossible to write after all.