Drools (used to be Drools then JBoss Rules then Drools again) is a very good open-source rules engine and definitely worth a look.
To my mind the best criteria to evaluate a rules engine are:
- Is it configurable (otherwise you'd just code it instead)
- Is it performant.
- How easy is it for non technical people to change rules
- How easy is it for new rule sets to be deployed (and old ones undeployed).
Several Rules Engines have Business Rules Management Systems (BRMS) and Drools is no exception. This makes deployment much easier for non-techies.
There is good support for Drools via docs and tutorials on the web and even via the IRC channel.
I've also used the Rules Engine that comes with IBM Websphere Process Server and found that to be quite good as well (although pricey).
