TortoiseSVN is a really easy to use Revision control / version control / source control software for Windows. It is based on Subversion. TortoiseSVN provides a nice and easy user interface for Subversion.
It is developed under the GPL. Which means it is completely free, including the source code. But just in case you don't know the GPL too well: you can use TortoiseSVN to develop commercial applications or just use it in your company without any restrictions.
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.

As a Subversion client, TortoiseSVN has all the features of Subversion itself, including:

* Most current CVS features.
* Directories, renames, and file meta-data are versioned.
* Commits are truly atomic.
* Branching and tagging are cheap (constant time) operations.
* Efficient handling of binary files.

* Easy to use
* Commit dialog
* The big picture
* Integration with an issue tracker
* Helpful tools