Why use a Rule Engine?
Short answers are as follows:
* The logic changes often
* The problem is just too fiddly for traditional code.
* The problem is beyond any obvious algorithm based solution.
* Domain experts (or business analysts) are readily available, but are nontechnical.
http://www.jbug.jp/trans/jboss-rules3.0.2/ja/html/ch01s02.html
Top 10
http://www.businessreviewonline.com/os/archives/2008/07/10_best_open_so.html
open source rule engine in java
http://java-source.net/open-source/rule-engines
open source rule engine in c#
http://csharp-source.net/open-source/rule-engines/sre