I'd like to find a few large .NET projects that have public source code available.
(Having said that, I now can't find the source download location :}
NHibernate is one of the biggest projects around and worth studying if this is the purpose of your question.
List of ASP.Net Open Source projects: http://wiki.asp.net/page.aspx/388/aspnet-open-source-projects/
Data & Object Factory has a sample application that they publish (Patterns in Action) to showcase design patterns in .NET. The application shows implemetation of the design patterns from the Gang of Four and .NET versions of the patterns from Head First Design Patterns. It's all done in C#, and shows UIs done in ASP.NET, WPF, and WinForms. I don't regret the subscription one bit (and it wasn't much at all).
If you don't mind spending a few bucks for something that gives you lots of really useful, heavily documented, working code that models a real-world application (a shopping cart for an online store), it's worth it (at least, in my opinion).
I heartily recommend them.
Since dependency injection is a hot subject at the moment check out the castle project
NeuronDotNet, an implementation of several types of artificial neural networks.