Hammurabi is a free and open-source rule engine written in Scala that tries to leverage the features of this language making it particularly suitable to implement extremely readable internal Domain Specific Languages.
This makes Hammurabi different from all other rule engines is that it is possible to write and compile its rules directly in the host language.
In addition, Hammurabi's rules have the important property of being readable even by non technical person. As usual a practical example worth more than a thousand words.
Detailed instructions on how to install and use the Hammurabi utility on your Mac are available HERE.
Hammurabi is cross-platform and it works on Mac OS X, Windows and Linux.