Microsoft have started open source project hosting named "Codeplex". In this place, you are find plenty of codes which you can use for free, play with it, contribute to project, can get members of projects, shortly you can find everything that is on sourceforge.net. Projects on Codeplex are Components, Control Toolkit, Extensions, Features that adds value to Microsoft's existing product families. If you are .NET developer, You should not miss this sites and projects on it.
Wikipedia definition for Codeplex
CodePlex is an open source project hosting website from Microsoft. It allows shared development of open source software projects. Its features include wiki pages, source control based on Team Foundation Server, discussion forums, issue tracking, project tagging, RSS support, statistics, and releases.
The initial beta launched in May of 2006, with the official release a month later in June. A new version of the website is released every three weeks adding additional features and updates. It has accumulated 1,642 projects as of June 9, 2007.