I had a look at this package and was fairly impressed with it up until I reviewed the licencing for the package, which may present some hurdles to those looking at incorporating Jess by itself into their own development. In summary from the download page ...

• Jess software, owned by Sandia National Laboratories, will be made available upon request at no cost to U.S. Federal Government Agencies for their own internal use. Sandia will also provide Jess to Universities, Academic Institutions, and other U.S. National Laboratories, for their own internal research and development, through a no cost, restricted R&D license. Any internal or commercial use of Jess requires a commercial license that can be negotiated as a running royalty or a fully paid up-front fee.
• Our commercial license will grant your company, the LICENSEE, a nontransferable, nonexclusive right to use Jess Software to create derivative works by embedding Jess into your product(s) and to copy and distribute Jess software as embedded into your Product(s).
• Note: Jess is not licensed under the GPL, the LPGL, the BSD license, or any other free software or open source license. Redistribution of the Jess source code under any free software or open source license is prohibited under this agreement.