Xinerama
The X11 extension that merged several monitors into one big surface, so windows could finally be dragged from one to the next.
Classic X treated multiple monitors as separate screens, :0.0 and :0.1. A program started on one stayed there; moving a window across to the neighbour simply wasn't possible. Xinerama, developed at DEC under the working name PanoramiX and contributed to X11R6.4 in 1998, let the server present all monitors as a single logical screen.
It was a blunt instrument. The layout was fixed when the server started, and every graphics card had to draw everything, which often disabled hardware acceleration. Programs could at least ask where the monitor edges were, so dialogs didn't land across the seam. RandR 1.2 took over the job in 2007, but the Xinerama queries live on as a compatibility API in nearly every window manager.