If you ever used any of the Microsoft Application blocks in your application, you'll love the Enterprise Library, released by Microsoft Patterns and Practices group. The library is composed of seven application blocks, all extendable, designed to work together and, what's best, very easy configurable with new Enterprise Library Configuration Console.
The library includes QuickStart samples (C# and VB.NET), application blocks' source code (with unit tests).
Excellent stuff. Go get it!