I have been using Subversion for a while (after having it recommended by my colleague Thomas), and I must confess I'm a happy user. Subversion is a revision control system, designed to supersede, and replace, the (maybe) more popular CVS.
Subversion (svn) is good for much more than collaborative development, as a single person can keep track of versions of her own documents/scripts/whatever. Usually you only want the last version of whatever you work with. But whenever you find yourself saving a version somewhere else, to keep it like that even if further changes are made to the "current" version, svn is your friend. Whenever you wish you had saved an earlier version of the stuff you're working with, you're missing (know it or not) svn.