MacPaint was one of the first big "wows" of the graphical UI. Before the early days of Mac OS, operating systems were strictly text affairs, and creating graphics was done mostly by writing code. But MacPaint helped to change all of that, putting image creation in a graphical user interface (creating standby design ideas like the "marching ants" selection indicator), and allowing those images to be used in other programs and applications. Now, Apple has donated the MacPaint source code to the Computer History Museum in Mountain View, California.
Andy Hertzfeld (who writes about the MacPaint code here) is perhaps the one most responsible for the donation -- he hunted down some original floppy disk copies of the app, and then installed it on a networked Lisa computer to obtain the source code, and then came upon the idea of getting it donated to a museum so everyone could see it. After poking around Apple for a few years, he finally talked to Steve Jobs in January of this year, and Jobs fast-tracked the approval process so the donation could happen today.
Very cool story, and it's excellent to see a little piece of Apple (and computer) history enshrined in a museum. You can get both the MacPaint and QuickDraw source code right off of the museum's website.