Interestingly i have worked on a proprietary rule engine, which is an excel based tool and seen the way its used in insurance industry. Essentially this means the business users can continue defining the rules in the excel sheet, with one condition customized excel sheet and change the rules as applicable and deploy to make the whole java application work based on the newly defined rules.
Abacus Formula Compiler for Java is an open source rule engine excel/open office based engine. Tried a sample works like a cool machine, performance details need to be analysed though.
1.Key selling point excel based tools are best welcome by business users.
2.Very simple and easy integration with java code.
3.Useful in industries such as banking & insurance for actuaries, underwriters for rate calculation and etc.
4.Open source. ☺