I am looking for a sample application which shows off good to great use of architectual patterns, so far I found
The problem with the above is that the requirements to get this up, make it a nightmare to work with.
My requirements list is it should be simple to get up and running, covers the basics and does need to be in a .NET language.
May not meet your need for simple to get going but take a look at
Just reading about their architecture is a good start.
Grab a copy of Eric Evans DDD book for reading material as well.
I found out about the The Northwind Starter Kit through this great book by Dino Esposito, et. al. Uses a layered, MVC Architecture and is very easy to setup. Go to source code tab and download ready made Visual Studio solution.
.NET StockTrader example application from Microsoft, showcasing an enterprise SOA architecture.
I found Eric Evans DDD book to be the most inspiring book on Enterprise Application Architecture I have read so far. Reading this book, I have learnt more about Software Architecture than by studying any sample application.
As a side note, I wouldn't constrain myself to a .NET language. E.g. if you know C#, you'll find Java easy to read.
Saying this, there is a big community around DDD and they have a good sample application (in Java) on http://domaindrivendesign.org/.
NDepend documentation comes with some pretty cool online blog posts, articles and white books concerning architecture of .NET code:
Advices on partitioning code through .NET assemblies
Control Components Dependencies to gain Clean Architecture
Re-factoring, Re-Structuring and the cost of Levelizing
Evolutionary Design and Acyclic componentization