Microsoft's First Product: The Altair BASIC Source Code (1975)
Altair Basic Interpreter Source Code (1975) [pdf]
This PDF contains the original source code for Altair BASIC, Microsoft's first product, written by Bill Gates, Paul Allen, and Monte Davidoff in 1975. The 157-page document offers a rare look at the programming that launched the PC software industry, including handwritten annotations and the original comments.
Altair BASIC was the first product Microsoft ever made, and it marked the beginning of a revolution in personal computing.