Sites that has proven useful to me

The MVC framework site
The Model-View-Controller (MVC) architectural pattern separates an application into three main components: the model, the view, and the controller.
http://www.asp.net/mvc

The blog of Scott Guthrie
http://weblogs.asp.net/scottgu

Code Project
Source code and tutorials for software developers and Architects.
http://www.codeproject.com

NHibernate Forge
NHibernate is a mature, open source object-relational mapper for the .NET framework.
http://nhforge.org

AutoFac
Autofac is an addictive Inversion of Control container for .NET 4.5, Silverlight 5, Windows Store apps, and Windows Phone 8 apps.
http://autofac.org

Stack Overflow
Stack Overflow is a question and answer site for professional and enthusiast programmers.
http://stackoverflow.com

Scott Hanselman
http://www.hanselman.com

CQRS
An introduction - CQRS stands for Command Query Responsibility Segregation and is a model for highly scaleable sites.
http://martinfowler.com/bliki/CQRS.html - by Martin Fowler
http://msdn.microsoft.com/en-us/library/jj554200.aspx - by Microsoft Patterns & Practices

C# Language Specification
The C# Language Specification is the definitive source for C# syntax and usage.
http://msdn.microsoft.com/en-us/library/vstudio/ms228593.aspx

ASP.NET
The official ASP.NET website.
http://www.asp.net