One of our favorite .NET books is "Framework Design Guidelines" by Krzysztof Cwalina and Brad Abrams.  In fact, it's required reading here at LifeCycle, and we try to check all code with FxCop to adhere to as much of the guidelines as possible.   Here is an excelent video presentation on many of these principals by Krzysztof.  It's a little long- 3 hours- so you may want to break up your viewing a bit,  but this guy is an absolute genius and what he has to say can go a long way in making or breaking your applications and APIs. 

Last month I mentioned a couple upcoming tools for "next generation unit testing", but it should come as no suprise that Microsoft is also working on a similar tool of their own. From the maker of MbUnit, Pex is a unit testing framework that auto-generates test inputs for you. So, if you write a test like this:

[PexTest]public void MyTest(string input){...}

Then Pex is smart enough to generate tests for the full range of values for "input":

MyTest(""); MyTest(null); MyTest("somevalue");

Add in auto-correction, and the result is a pretty impressive test-driven development story. Unfortunately, this is currently only a Microsoft Research project. Hopefully it sees the light of day sometime soon (and not just for Team System!).