Bill Budge has posted to github the source code to his seminal hit Pinball Construction Set, according to this Y-Combinator item.  Some sharp eyes in the comments thread noticed that although the code is for the Atari 800 version of the program, it was developed on an Apple II, a common practice in those days and made easy by the fact that both platforms use a 6502 processor. According to one of Bill’s recent tweets: “The Atari 800 had a raster mode that was very similar to the Apple II Hi-res graphics. The code had minimal differences.”
Bill has also tweeted that he intends to go through and comment the code, which you can find here.
(HT: Sean Fahey, image source: The Digital Antiquarian)