There a new and very interesting project on CodePlex today. It's a "bridge" that allows Subversion clients (like TortoiseSVN) to work against a TFS server. This project has been created by the team that develops CodePlex itself due to the popularity of Subversion clients. This bridge works at the protocol level by transforming the SVN protocol to the TFS protocol and back. It's still in it's infancy but it's something worth keeping an eye on.
Here's the project: http://www.codeplex.com/SvnBridge
Keep in mind, of course, that using it still requires a TFS CAL, sorry - no free lunch here 