I recently needed a simple rules engine for a project so I wrote my own (using Windows Workflow with a re-hosted workflow editor for the rule creators). It does not utilize any forward/backward chaining of rules like most of the other rule engines, just executes the rules from top to bottom against the desired .NET object. It's simple but it worked for what I needed. It was difficult enough to create so I decided to license it. Check http://www.enigmasupercomputing.com/RulesEngine for tutorials and demos and a 30-day trial.
