Drools.NET

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.
Core Rules Engine Features
Drools.Net 3.0 supports the following features:
    * Full Rete Implementation with high performance indexing
    * Field Constraints
    * Literal, Bound Variable, ReturnValue and PredicateConstraints
    * Conditional Elements
    * And, Or, Not, Exists and Eval
    * Agenda Management
          o Conflict Resolution (Salience+Depth)
          o Agenda Groups
          o Activation Groups
          o Agenda Filters
          o No loop for recursion control
    * Truth Maintenance with Logical Assertions
    * Temporal Rules (allows rules to fire based on time requirements)
    * Dynamic Rules (runtime addition and removal of rules)
    * Event Model
          o Working Memory Actions
          o Agenda
    * Functions
    * Global Ddata
Debugging/Deployment Features:
    * Drools.Net supports debugging of rules by putting breakpoints in consequences in a rule file.
    * Dynamic runtime compile and use.
    * Precompiled Packages.

Go to http://droolsdotnet.codehaus.org/

See also these related projects.