TortoiseSVN is a really easy to use Revision control / version control / source control software for Windows. It is based on Apache? Subversion (SVN)?; TortoiseSVN provides a nice and easy user interface for Subversion.
It is developed under the GPL. Which means it is completely free for anyone to use, including in a commercial environment, without any restriction. The source code is also freely available, so you can even develop your own version if you wish to.
Since it's not an integration for a specific IDE like Visual Studio, Eclipse or others, you can use it with whatever development tools you like, and with any type of file.