In addition to the other answer, when I copy a project from my laptop
(doesn't have SourceSafe) back onto my desktop (does have SourceSafe), I use
Beyond Compare, from Scooter Software. It doesn't know anything about Visual
Studio .NET, but its directory and file compare is excellent, including the
file viewer, which is very good at synchronising two files side by side.
There are plenty of tools on this theme: VSS, CVS, PVCS, Perforce,
Bitkeeper, Subversion, ClearCase, to name just a few.
CVS and Subversion are free, and there are plenty of tools around CVS
(Tortoise CVS is my favorite).