John,
I've used quite a few over the years. I've basically settled on using two, either Subversion or Git. Subversion is arguably the simpler of the two. On single-developer projects it works out nicely because no "server" is actually necessary (although Git doesn't need one either, but anyway...). Subversion is simple because it basically tracks commits to your central repository (this can be a directory on your system, not necessarily a server) by number in sequential order. Additionally, on Windows, TortoiseSVN is a very powerful Subversion client that can eliminate any need for command-line use.
Git is a form of distributed version control, but, for a single developer, this distinction might not be overly important. Git is, in my opinion, more powerful because "branching" the code is drastically simpler. Branching is central to the Git workflow. Basically, you'd create a local branch, make your changes, merge them back to the "master" branch, and commit them. Git's learning curve is a bit steeper than Subversion, so it might be better to start with Subversion. Additionally, Git's tracking of your code via hashes rather than a simple, sequential number can be intimidating at first.
There are some great books available for free for both these solutions:
Subversion: Version Control with Subversion
Git: Pro Git
I wouldn't suggest any other version control systems at this time. Subversion was designed to be a "better" CVS, so I would suggest avoiding CVS. Mercurial is often offered as an alternative to Git, but I've never been a particular fan of it. There are a few proprietary ones, but I think maybe Perforce is the only one that I've actually enjoyed using.
If it makes any difference, Simply Fortran itself is developed using Git as source control, but it had previously used Subversion.
Simply Fortran doesn't directly integrate any version control system, incidentally, because we've never had any users request that feature.
Jeff Armstrong
Approximatrix, LLC