SRE (Simple Rule Engine) is a lightweight forward chaining inference rule engine for .NET. Its 'simple' because of the simplicity in writing and understanding the rules written in XML, but this 'simple' engine can solve complex problems.
I'm using this on a project right now that has to run in a sandbox imposed by (.NET CAS for SharePoint 2010) and, I'm impressed at how lightweight and straight to the point it is. Another huge plus is having the ability to store your rules wherever you want. For me this makes a huge difference as SharePoint lists and document libraries are the staple storage solution for most of the configuration files I reference. Big ups!!! More Examples and documentation please :-)