Drools
Drools is an augmented implementation of Forgy's Rete algorithm tailored for the Java language. Adapting Rete to an object-oriented interface allows for more natural expression of business rules with regards to business objects.

JEOPS - Java based Rule Engine
JEOPS is a Java based forward chaining RULE ENGINE. This Rule Engine is used to power up the business process by rules in Java Application Servers, client applications, and Servlets.

JLisa
JLisa is a powerful framework for building business rules accessible to Java and it is compatible with JSR94 V, the JavaTM Rule Engine API. JLisa is more powerful than Clips because it has the expanded benefit of having all the features from common lisp available.  These features are essential for multi-paradigm software development.

Mandarax
Mandarax is an open source java class library for deduction rules. It provides an infrastructure for defining, managing and querying rule bases. Rule bases can be made persistent using the XKB module.

OpenRules
OpenRules is a full-scale open source Business Rules Management Framework. It efficiently uses the power of MS Excel, Eclipse IDE and open source Java libraries to create, deploy, execute, and maintain different rule engines with complex business logic controlled by business analysts.

Prova
Prova positions itself as a rule-based scripting (RBS) system for middleware. The language breaks new ground in combining imperative and declarative programming.