subversion seems to be trying to be a substitute for CVS but i haven't found it so popular, at least now. perhaps it's not so mature yet? and it's not compatibale with CVS yet?
for totally free using opensource you should go for CVS.
on windows: CVSNT for server and various clients: TortoiseCVS, WinCVS... these can be downloaded from sourceforge.net
I'm using CVSNT with tortoisecvs now.
Added after 4 minutes:
btw, if you could find any good software, just lemme know. i'm very interested in these stuff
As time goes by there will always be happy and unhappy people using the
same software, be it OSs, Games, WebSurfers and version control.... so,
since it's free and work reasonably well to manage different kinds of
documents I use CVS (wincvs)...
I have a mix of binaries, excutables,PDFs, text files, gifs, scheamtics, etc... and
it works well enough.