http://herzberg.ca.sandia.gov/jess
Jess 6.1 includes significant new functionality (arbitrary nested conditional
elements,) ease-of-use features (new APIs for logical dependency and working
with definstances,) enhancements (a new format implementation, more control
over rule firing during queries,) and bug fixes (deadlocks and memory leaks
especially.) Jess is available at no cost to students and government users,
and can be licensed for commercial use; see the web site for details.