tvtwm
Tom's Virtual twm — twm with a desktop larger than the screen and a little map to pan around it.
Built on twm, it was one of the first Unix window managers with a virtual desktop. Unlike later workspace switchers, it offered a single huge surface several screens wide, with the monitor acting as a viewport you slid across it. A panner window showed the whole area in miniature, and you dragged inside it to look somewhere else.
To pull this off tvtwm slipped a giant window between the root window and the applications, tagging it with the __SWM_VROOT property. That broke anything convinced it was drawing on the root: screensavers, xsetroot and background setters all ended up behind the virtual root, invisible. The fix was a header file, vroot.h, which programs included to go looking for the fake root — a patch that xscreensaver still carries around today.