Colossal Cave Adventure
The first text adventure, written on a PDP-10 in the mid-1970s and copied to every ARPANET machine with a spare terminal.
Will Crowther, a BBN programmer who had worked on ARPANET's IMP routers, wrote it around 1976 based on his caving trips in Kentucky's Mammoth Cave. You typed two-word commands — GO NORTH, GET LAMP — and got back a description of the next room. Don Woods at Stanford found the code in 1977, got permission to extend it, and added trolls, treasure and scoring.
The game travelled across the network as a file and cost productive hours at every university that had it. It led directly to Zork, gave the genre the name "adventure", and indirectly to MUD. The magic word XYZZY still works as an Easter egg in a surprising number of programs.