TortoiseSVN is an easy-to-use SCM / source control software for Microsoft Windows and possibly the best standalone Apache™ Subversion® client there is. It is implemented as a Windows shell extension, which makes it integrate seamlessly into the Windows explorer. Since it's not an integration for a specific IDE you can use it with whatever development tools you like.
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.