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.
Enterprise Library 4.0 for Visual Studio 2008 is available for download now:
http://msdn.microsoft.com/en-us/library/cc467894.aspx