From the site
The patterns & practices Enterprise Library is a library of Application Blocks designed to assist developers with common enterprise development challenges. Application Blocks are a type of guidance, provided as source code, that can be used as-is, extended or modified by developers for use on enterprise development projects. Enterprise Library features new and updated versions of application blocks that were previously available as stand-alone blocks. All blocks included in Enterprise Library have been updated with a particular focus on consistency, extensibility, ease of use and integration.