Here is a real gem: The computer history museum has released the source code of photoshop 1.0 for classic the macOS with permission from Adobe! 8)
Here's the article with the link: http://www.computerhistory.org/adobe-ps-source/ (http://www.computerhistory.org/adobe-ps-source/)
The first version of photoshop was mainly written in ObjectPascal (about 75%) and in motorola 68000 assembler for about 15%, and the rest
is data. I have looked at some of the files, and they look really neatly written. I'd say, check it out, just for nostalgia. :)
It was mainly written by only two people, and has about 128.000 lines of source code, quite interseting if you look at this youtube clip showing what the very first version could do: http://www.youtube.com/watch?v=Bhf8q3THToI (http://www.youtube.com/watch?v=Bhf8q3THToI). Makes you wonder why some modern software pacakges have over millions lines of code and are so bloated and inefficiently programmed! ;D (any current adobe / autodesk / <any big software company> program).
The only thing missing is the source code for macAPP, wich was the application framework from apple.