TortoiseSVN is a Subversion (SVN) client/server for Windows.  It adds some little marks like check boxes and ! to your windows icons to show you which files are out of sync with those on the server.  It adds a right-click menu to access it’s functionality.  You don’t have to remember any of the svn command line parameters.

Also useful is AnkhSVN for Visual Studio which adds similar functionality to Visual Studio’s Solution Explorer.  It is a drop in replacement for Microsoft’s Team Foundation Server.