A couple months ago, I learned about Microsoft's official process & support for community, open source projects centered around www.codeplex.net. At that point, the site was pretty empty, but it's turned into very active development repository. A profusion of Windows Live Writer plugins are being developed in this VS Team System derived environment. There are also some very interesting projects:
It's hard to say how the environment will compare to other open source repositories I've used like sourceforge and mozdev, but the what's new RSS feed is better designed than the either of those systems at least! Check out the Codeplex RSS community with the OPML Generator.