The Alto was a personal computer developed by Xerox at the Palo Alto Research Center (PARC) in the early 1970s. It was revolutionary, being one of the first computers to feature a graphic display (a monitor with a 606x808 black and white display), keyboard, mouse and had, for the time, a beefy amount of memory (128 KB) and hard disk storage (2.5 MB). It also introduced the concept of a computer desktop and featured the first Graphical User Interface. The Alto was never sold commercially; instead, about 1,500 manufactured and made available throughout Xerox.
You can download Alto source code from the Computer History Museum’s website which includes the OS and a range of software and utilities developed for the Alto such as PUP (PARC Universal Packet) a protocol for internetwork communications, Bravo, the first WYSIWYG editor, and four different programming languages the Alto supported: BCPL, Mesa, Smalltalk and Lisp.