CVS is quite cool, it's the system used by huge number of open source
projects and supports (to the best of my knowledge) everything you can
so with, say, Visual Source Safe, but with a lot of cool advantages
too. Here's a link to some screenshots of a windows front end:
http://www.wincvs.org/shots.html. It's also open source, so there's
room to improve. There are really a ton of other interfaces, though,
including web, java, Tcl/Tk, Mac, Linux (Gtk or Qt), etc. Check out
www.cyclic.com.
--JRZ