I use Subversion. It's quite popular, though version control software can be a little tricky to setup. Using a tutorial like this one can help though.
I'm not an expert at VCS/SCM, but I know the Mac now has some really awesome GUI svn (subversion) clients. If you haven't already, check out http://versionsapp.com/ and Cornerstone. Even Coda has built in SVN support. So going with SVN is probably a safe bet. Cheers~
Another vote for Subversion. Even though I don't use Dreamweaver that much it's still nice to know that CS4 can work with SVN. Also, the Versions app is very nice to work with, I've only been using it for a few days but it takes a lot of the hassle out of using version control.