CodePlex.com by Microsoft
Well, Microsoft has stepped even further into open source. Some time ago they have opened an open source community portal – codeplex.com – a nice place for those who are developing open source projects based on microsoft technology and for those who have interest in such projects. So grab a free copy of Visual studio Express 2005 and go coding. I have been using Visual Studio Express 2005 for a while and find codeplex combined with msdn quite useful.