Open Source Rule Engines in C#

Drools.NET is a Business Rules Engine (BRE) based on Charles Forgy's Rete algorithm. Developers can now exploit a powerful Rule Engine through a completely managed .NET code base! Drools.NET is based on Jboss Rules, and comes with all the features of that Rules Engine.

NxBRE is the first open-source rule engine for the .NET platform and a lightweight Business Rules Engine (aka Rule-Based Engine)

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.