I used Subversion (with TortioseSVN and RapidSVN) when I was building Strine (including installing it on the team's machines and training them how to use it) and completely loved it.
Good version control on a project has always been important in my eyes after years of working in desktop publishing roles, where you go through any number of iterations and edits on a document.
I created my own version control system, where I label all of my documents with date, series label and owner initials, and sometimes time (e.g. File name 310310JTa 1745.ppt). I will save a new version any time I'm about to make a new set of significant changes (especially deleting), because you are guaranteed to have someone want that [page, chart, section] back right at the last minute. In a deadline driven environment, you don't want to have to recreate things.
Anyway, this system, although useful meant I chewed through space on my USB keys and after enough iterations on a project, it was a pain to scan through loads of files to look for the change you wanted to undo. When I started using SVN, it was like a dream come true - my OCD need for versioning could be satisfied easily! And across a whole team of people! Awesome tool.
Anyway, now I'm back to working on a solo project (LIF site redesign), I've reverted back to filling up my USB keys with versions of files. I don't like this system - I feel like I've flown business class and now I'm being forced to sit back in Economy again.
I was talking about SVN at the Web Blast evening last week with a gentleman named Sam, who told me all about other version control systems out there, as well as options for remote hosting. He suggested if I want to get into developing that this would be a good idea to set one of these up, as it would be an easy way to show people in interviews what my code actually looked like. I just liked the idea because I could work on my site build anywhere (including at work) and upload changes regularly - and save my usb keys!
Stack overflow had a useful thread on this exact topic. Tomorrow I'm planning to check out Unfuddle and Beanstalk and see which free hosting plan suits me best. They both also support Git, a different VCS. I think I'd like to try something else new, to add another feather in the cap.