Subversion is the an open-source version control system - in essence a successor to CVS. It fixes the biggest issues with CVS, introducing such things as atomic commits and support for file and directory renaming. I've been using it for a couple of years and have found it very solid.
My colleague Mike Mason has written the Pragmatic Programmer Book on Subversion. While you can get a detailed book online, Mike's book gives you an excellent introduction to subversion and how to use it. Buy lots of copies and we won't have to give him a pay raise.
One of the big ways I use subversion is to handle MultipleDesktops. I keep all my working files in Subversion and use updates and commits to keep everything in sync.
On the basis of my informal conversations with my colleagues, Subversion (abbreviated to SVN) is a better system than all the commercial tools except Perforce. So if you're using something else, you may want to consider a switch.
As someone who uses version control all the time, I think it's something that can grow into more areas of computer use. Other than software developers, few computer users use version control. Yet as software developers know, version control is a great mechanism for collaborative work, allowing multiple people to work together on a single software system. What would be the benefits of version control being more widely used? Most applications that people use have little capabilities to do diffs and merges. If applications were more aware of version control then I think we would see more interesting things being done with it. I hope that a solid, usable, open source tool will spread the usage of version control to a wider audience - after all we are no longer short of disk space.