The Computer History Museum has recently posted the original source code for MacPaint and QuickDraw! Apple has given them permission to publish them both, and they're well worth taking a look at if for no other reason than to realize just how good we programmers have it today.
The source code is a combination of 68k assembly, resource files, and Pascal, and all of the code was written by the incomparable Bill Atkinson, one of the early heroes of Mac programming and author of Hypercard (among many other things).
As an interesting aside, Bill stopped programming several years ago to focus on photography, but started programming again after the iPhone SDK came out so that he could create PhotoCard.
If you're not really familiar with who Bill is, you might want to read some of the stories on Folklore.org, but be warned, it's easy to lose track of time at that site if you have any interest in Apple history.
Note: One question that occurred to me looking at this source code was "how could Bill could have coded this back then?" These were written before the Mac shipped, and the Mac shipped with only 128k of RAM, which is smaller than the MacPaint Pascal source. You couldn't have opened some of these source code files on an early Mac due to their size. The answer appears to be that they were written on a Lisa, which shipped with a meg of RAM. That was some crazy amount of memory for the day and part of the reason the Lisa cost $9,999 (somewhere between $19k and $40k in today's dollars. I do seem to recall reading, though, that the early versions of QuickDraw (aka LisaGraf) were written on some model of Apple 2, though.