Herman Ventor, the brains behind FxCop's assembly metadata reader, has just released the next version as an open-source project over on CodePlex.
The Common Compiler Infrastructure (CCI) is a set of components for reading and writing CLR assemblies and was developed as an alternative to Reflection. This is an amazing project and if you've ever been frustrated by the limitations of Reflection and Reflection Emit - I would highly recommend downloading and checking it out.
Find out more at: http://ccimetadata.codeplex.com/