CodePlex is Microsoft’s open source project hosting web site. Start a new project, join an existing one, or download software created by the community.

As you would assume its a lot of projects written using MS technologies like ASP.NET.

Link:

Check out these posts too:

  1. A Short Guide To Open-Source And Similar Licenses
  2. Open Source is a valuable resource
  3. Love's open source
  4. Open source Knowledge Base hits the mark
  5. Open Source CMS
  6. 5 Steps to Winning Any Client Project
  7. Microsoft confirms 'Origami' project