What is the history of SpamAssassin?
SpamAssassin was originally imported into Sourceforge on 2001-04-20; this was pretty much the very first code for SpamAssassin. The changes can be viewed through the CVS repository on Sourceforge; the link is on the "hacking" page.
Before that, there was filter.plx; this is described at http://SpamAssassin.org/old/prehistory .