XVideo (Xv)
The X11 extension that let the graphics card scale and colour-convert video so the CPU didn't have to.
Video is stored as YUV, screens want RGB, and the window is rarely the same size as the film. Doing the conversion and scaling on the CPU and then pushing finished frames through Xlib was too much for 1990s hardware. XVideo, developed by David Carver at DEC early in the decade, lets the client hand YUV frames to the server, which passes them to the card's overlay or scaler.
XFree86 4.0 in 2000 brought driver support on ordinary PC cards, and -vo xv became the standard answer to "why is my video stuttering". Overlays had quirks: a colour key, usually magenta, marked where the video should show through, and screenshots captured only the magenta. Compositing desktops and GL-based playback have since taken over.