TortoiseCVS is an open source tool front-end for CVS that integrates CVS items with the Windows Explorer. Checked out items appear in your the explorer, are available from the file open dialogs, show their state with colored overlays, with allowed actions showing up in the context menu.
You’d wish there were a way to do explorer integration with FTP/SFTP/SSL, and there is, with KnoWare‘s Internet Neighborhood Pro (which has ftpNetDrive’s drive mapping ability integrated now). It’s a great program, but $40. Still well worth it, relatively speaking. You’d think there be an open source equivalent, but well, this is the Windows world we’re talking about, I suppose.