Source code for all Infocom text adventure classics on github.
According to a story on Arstechnica the source code of every Infocom text adventure game has been uploaded to GitHub
There are numerous repositories under the name historicalsource, each for a different game. Titles include, but are not limited to, The Hitchhiker's Guide to the Galaxy, Planetfall, Shogun, and several Zork games—plus some more unusual inclusions like an incomplete version of Hitchhiker's sequel The Restaurant at the End of the Universe, Infocom samplers, and an unreleased adaptation of James Cameron's The Abyss.
Some very interesting retro stuff indeed.