Microsoft releases MSDOS 1.1 and 2 source code. Also Word for Windows source.

[FAST6191]

http://www.computerhistory.org/press/ms-source-code.html
Sadly not in a more recognised open source license but a more "you can read it and fiddle with it on your own machines for non commercial purposes" affair, full license http://www.computerhistory.org/ms-dos-early-source-code/agreement/ . To that end and for practical purposes you are considerably better off looking up something like Freedos, dosbox or possibly even something in the reactos line of thought.
However if you ever wanted to see the things that MS started with then here is your chance.

Previously Apple, IBM and even some of the source code for the very first version of Adobe Photoshop have been released.
http://www.computerhistory.org/atchm/category/from-the-collection/


Source
http://www.theregister.co.uk/2014/03/25/microsoft_releases_operating_system_source_code/

 

[Taleweaver]

"Version 1.1 fits an entire operating system – limited as it was – into only 12K bytes of memory, which is tiny compared to today's software," said Len Shustek, CHM Chairman.

Yeah...I dare say today's operating systems are a bit larger than 12 kilobytes, all right.

 

[VashTS]

back then 1.44MB was very large. good ol' days of computing, just waiting for the future. now we are here and i love every minute of it.