Updated Sept 19 2008: added Ruby/EventMachine links
Updated Sept 19 2008: more links for Ruleby

Not a major category among the many rule engines implemented in Java and other languages. In this case, I've pushed the definition of 'engine' to the limits so I can include some of the more solid and interesting implementations.

Ruleby is probably the most popular and mature Ruby rule engine at this time, in part because it is well-implemented and written in pure Ruby. It is also a clean implementation of the forward-chaining RETE algorithm. No backchaining.

Yet another reason to take notice of Ruleby is that founder Joe Kutner did a really excellent presentation on rule engines at the Ruby Hoedown 2008. It's about a half-hour long and one could spend many hours slogging though web sites and learn less about rule engines. Highly recommended.

Ruby Rools is not based on RETE, but has a rule scheduling mechanism.

There are also some interesting Ruby workflow engines, such as the OpenWFEru workflow engine and the Ruby Event Machine providing the framework for an event-driven I/O engine, apparently similiar to Twisted, an event framework for Python.