I've used CLIPS, Blaze Advisor, ILOG Jrules, Jess and OPSJ. Depending on the application and money available and size of the project and specific requirements, I could reccommend any one of them. Blaze Advisor and ILOG JRules are quite expensive but they seem to be the only two 100% Java inference engines with all of the developer tools required for an enterprise development. Jess and OPSJ both are quite good, but OPSJ is the fastest thing on the planet. I posted an earlier comment with links.