Enterprise Library 6.0, a collection of reusable software components for developers working in large environments, for the first time in three years. The Library helps with "cross-cutting concerns" like logging, validation, data access, exception handling and so on. Microsoft calls these software components "application blocks". They contain not just source code, but also documentation and test cases. The latest version is called Enterprise Library 6; the previous full release, Enterprise Library 5, came out in April 2010. Check it out here.
Microsoft has released the new version of their Enterprise Library.