Archivist Jason Scott is posting the source code for a number of old games to Github. The collection ranges everywhere from Leisure Suit Larry and Zork from the 1980s to Descent from 1995 and Mech Commander 2 from 2001.
Who knows how long these repositories will last, but as long as the collection is online it's a treasure trove of game history. Check out the repository list at https://github.com/historicalsource.
