Open Source:

• Drools written in Java, compatible with JSR94.
  • Give Your Business Logic a Framework with Drools
  • Using Drools in Your Enterprise Java Application
• JLisa Compatible with JSR94. A Clips-like Rule engine accessible from Java with the full power of Common Lisp
• OFBiz
• Mandarax
• 一大堆Java based rule engine
• Pychinko: Rete-based RDF friendly rule engine
• RuleCore, for Python
• Ruleby, for Ruby

Commercial:

• Jess
• ILOG JRules
• Rules4J

Related articles:

• Java规则引擎与其API(JSR-94)
• A Design Pattern for a Rule Engine(PDF)

怎麼現在的Rule engine幾乎都是Java寫的…@_@