Allegro
A C game programming library that gave a generation of DOS hobbyists graphics, sound and keyboard input without making them write their own interrupt handlers.
Shawn Hargreaves started it around 1990 on the Atari ST, and the name began life as an acronym: Atari Low-Level Game Routines. The real breakthrough came with the port to DOS and the DJGPP compiler, where Allegro handled VGA modes, sprites, sound cards and timers for anyone who simply wanted something moving on screen. The licence was "giftware": do what you like, but feel free to send something back.
Allegro 5 arrived in 2011 with a rewritten, event-driven API on top of OpenGL and Direct3D, leaving it with more in common with SDL than with its own DOS past. Hargreaves himself later ended up at Microsoft, working on XNA.