Most .NET developers should know about Microsoft Enterprise Library. The current release is 4.0 as of May 2008.
As quoted from the website, the patterns & practices Enterprise Library is a collection of reusable software components (application blocks) designed to assist software developers with common enterprise development challenges (such as logging, validation, data access, exception handling, and many others). Application blocks are a type of guidance; they are provided as source code plus documentation that can be used “as is,” extended, or modified by developers to use on complex, enterprise-level line-of-business development projects.
Even if you use other frameworks or build your own framework I would suggest that you download the library and study the source code. You will surely learn something on enterprise design patterns by going through the source code. The design of application blocks encapsulates Microsoft recommended and proven practices for .NET application development.
There is also a community available at http://www.codeplex.com/entlib.