Here are some of the open source projects/tools developed in .NET which are available on codeplex:
1) Silverlight Audio Player
2) Managed Extensibility Framework – New in .NET framework 4.0, this framework can be used for creating extensible applications and components.
3) MEF Visualizer Tool – Helps to see what is going on inside the MEF Composition Container. A visualization tool for MEF.
4) .NET Gadgeteer – a rapid prototyping system for pluggable modules in place of plugging in electronic devices. The end user site is at http://netmf.com/gadgeteer. To learn more, check out a Channel 9 Video on it.
Check out more on CodePlex.