I'm looking for some large open-source .NET (especially C#) projects to participate in as a developer, I'm interested in both web and desktop applications. I want the project to have a large number of members to get the feeling of working with a team.
Thanks for your suggestions.
Related: Open source C# projects that have high code quality?
MediaPortal is an Open Source application ideal for turning your PC in a very advanced Multi-Media Center / HTPC. MediaPortal allows you to listen to your favorite music & radio, watch your video's and DVD's, view, schedule and record live TV and much more.
MediaPortal is built on the Microsoft .NET framework using C# and currently has a growing international community of over 60,000 users.
You could have a look at the Castle Project, they seem to do a range of .Net things. Here's the getting involved page.
You'd want to participate in projects that are emerging and actually filling up major gap in .Net framework.
I'd recommend http://trac.caffeine-it.com/openrasta/ (an advanced REST Framework for .Net) and NServiceBus (http://nservicebus.com/), an awesome service bus implementation with a fast growing community.
I want to add the huge one, Mono (Most parts are developed in C# (Discussion)).
For web-applications there's ASP.NET Open Source Projects.
It has multiple categories: Blogs, Content Management Systems, Controls, Toolkits, Frameworks, Forums, Galleries, Starter Kits, Wikis, Miscellaneous and Additional Projects links:
http://asp.net/community/projects/
http://www.codeplex.com/
http://csharp-source.net/
http://www.nopcommerce.com/ - Open source asp.net shopping cart application
http://www.codeplex.com/nexus - Open source igoogle/pageflakes like application