I would suggest either SVN (subversion) or CVS.
Subversion is an "update" to CVS and is still being actively worked on... CVS has been around for over 20 years and is extremely stable. SourceForge.net runs off of CVS, and some other large repositories out there do as well, then on the same token, most "new" projects out there in the open source that have their own thing seem to be running off of SVN.
I personally use SVN for all of my personal stuff, all of my stuff at my "day" job, and managed to convert everything to SVN at my previous job. I have been extremely pleased with it (unlike SourceSafe), and found it easy to work with.
SVN can be installed on Windows, Linux, or Unix... I am running mine on a Windows machine (for the repository host, client can be installed anywhere), though I imagine you could get better security "faster" by using Linux or Unix. I just have 0 linux/unix experience and while I *want* to start learning it, just too busy to give it a whirl.
There are several other solutions out there that don't look half bad... just when you compare them to an open-source project that, just like DNN, has a large community... well, you can't really beat that.