This is not new but I just realized today that CodePlex supports TortoiseSVN. I've been using Subversion for my home projects with TortoiseSVN and VisualSVN (a plug-in for Visual Studio).
This does not mean that Codeplex is using Subvesion as repository. They are still using Team Foundation Server as repository, but they added support for SvnBrigde, a tool that allows Subersion clients to connect to Team Foundation Server.
So far, I've been using Visual Studio Team Explorer with Codeplex projects. Team Explorer is an open source client for Team Foundation Server. It works fine, but I rather using TortoiseSVN/VisualSVN.