Subversion

Subversion is a "source code repository" system. In other words, "a way of keeping track of source code as it evolves."

Subversion is very well documented; Steven recommended this online version of an O'Reilly book, and I've found it quite useful.

The svnX application for OS X is a very well-crafted GUI front-end for Subversion that makes day-to-day use much easier.