CodePlex is an open source project hosting website from Microsoft. It allows shared development of open source software. While CodePlex encompasses a wide variety of projects, including SQL, WPF and Windows Forms-related projects, major activities center around the .NET framework, including ASP.NET, and Microsoft's collaboration server, SharePoint. The most prominent and used project that was born inside CodePlex, the AJAX Control Toolkit is a joint project between the community and Microsoft.
About (from CodePlex Community at www.codeplex.com)
“CodePlex is Microsoft's free open source project hosting site. You can create projects to share with the world, collaborate with others on their projects, and download open source software.”