Picks of the Week
This is one of the most useful links you can find - The complete listing of the GOF (Gang of four) patterns with downloadable C# code http://www.dofactory.com/Patterns/Patterns.aspx. You would be finding many patterns already in the MS patterns and practices site ( like Observer, MVC, Singleton), but this one is the complete set with a taxonomical listing - a must see!
My second pick - Scott Mitchell's article series on Data Structures in C#. I guess the six part series is now concluded and has been a fantastic resource. The articles cover many data structures like Trees, Graphs, Sets etc in complete depth and the writing is very lucid too.