The Patterns & Practices team at Microsoft provides a collection of resources to help developers apply best practices to the development, deployment and operation of enterprise applications. Guidance comes in four primary forms: Software Factories, Application Blocks (including Enterprise Library), Reference Implementations and Guidelines.
Software Factories are configurable tools your architects and developers use to build applications. Application Blocks are reusable application components that implement software design patterns and are built with best practices for enterprise use. Reference Implementations are full, working applications that you can use as a starting point for your own applications, or simply use as examples. Guidelines appear as architectural and prescriptive guidance (wizards, how-tos, and documentation).
The Patterns & Practices code is available in source form so you can extend it, and there are active developer communities around many of the tools that offer additional guidance and support. Start using the tools today, join the communities, and become more productive as an architect and/or developer!