PHP Business Rules is an open source application layer for PHP. The logic is defined in rules and facts. Like most Business Rule Engines (BRE) it uses the Retealgorithm.
The Drools.NET is a .NET port for Drools, which is a java Rules Engine based on Charles Forgy's Retealgorithm. Drools.NET enables .NET developers/Users to exploit the powerful Rule Engine like Drools through a completely managed .NET code base.
Zerorule is a lite rule engine/expert system that implements the RETEalgorithm which can improve the speed of forward-chained rule system. It is written entirly in the powerful python language. it supplies both a script interpreter and simple CLI.