Drools on the JBoss website. I knew it was a rules engine, but apparently JBoss added more components to it to do different things.
A rules engine is a program that matches data to a set of rules. Usually you can specify those rules in a kind of mini programming language that is simple enough for non-programmers.
I've once worked on a system in which people could add rules to a table in a database. The rules would be things like "the value of this field is the sum of the values in these fields", or "the value in this field is a percentage of the value in that field", etc. You could use the system to define screens with fields, and rules to define the calculations done between those fields. So by defining the screens and rules you could for example make screens to calculate prices of items, with tax percentages etc.