XFree86-Bigfont
The XFree86 extension that stopped shipping hundreds of kilobytes of glyph metrics down the wire every time a program opened a large font.
When an X client opens a server-side font, the server answers QueryFont with metrics for every character, twelve bytes apiece. For a 256-glyph Latin font that is nothing. For a Unicode or CJK font with tens of thousands of glyphs it meant hundreds of kilobytes per open, and something like xterm with a few fonts configured started noticeably slowly even on a local display.
Bruno Haible wrote the Bigfont extension around 2000. Local clients received the metrics through a shared memory segment instead of the protocol reply, and glyphs with identical metrics were stored only once. Xlib used it automatically, so no application needed changing. The problem ended up disappearing anyway: Xft moved fonts to the client side, and server fonts became something only old programs asked for.