So it came as a surprise that Microsoft released MS-DOS 1.1 and 2.0 together with Word 1.1a source code to the The Computer History Museum. While these sources are far enough in past to be completely useless for anything competitive, I find them a precious part of history and very well worth checking.
MS-DOS source is particularly dear to my heart because it shows just how much functionality can be fitted in just 28 KB. Assembly code is quite annoying to follow (things tend to look a bit ugly when optimized) but it is well worth reading because of the comments. Not only they will help you understand code but, if you are old enough to have any DOS experience, they will also bring you on a memory lane stroll.
Since I was a stubborn WordPerfect fan at the time of Word 1.1a, its source didn't evoke same level of emotions in me. But I definitely found some parts amusing to see. Especially document titled 86fun.doc that lists some facts only programmer can think of as a fun.
I find both programs miniatures well worth checking and remembering. Thanks Microsoft!