Prism is a library that provides guidance and patterns for building composite client applications in WPF and Silverlight. It supports features like module discovery and loading, region-based UI composition, and multi-targeting of code between the two platforms. Prism 2.0 was recently released and introduced enhancements like improved module support, view discovery, and extensibility of the module catalog and loader. The library and documentation can be found on Codeplex and provides developers with tools for building modular, maintainable rich client applications.