Jess is a fast, light rule engine and scripting environment written entirely in Java. You can
build Java software that has the capacity to 'reason' using knowledge you supply in the form of
declarative rules. It is supplied as a programmer's library, making it ideal for embedding in larger applications. Jess includes development tools built on the Eclipse platform. It is free for academic use and can be licensed for commercial use."
Drools is a Rules Engine implementation based on Charles 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. Drools is written in Java, but able to run on Java and .Net.