There is a link to the Opentabs cookbook on this page: https://cwiki.apache.org/confluence/display/OFBIZ/FAQ+-+Tips+-+Tricks+-+Cookbook+-+HowTo#FAQTipsTricksCookbookHowTo-Groovy
This link does not work any more. It could be replaced with this link: https://web.archive.org/web/20161125105758/http://www.opensourcestrategies.com:80/ofbiz/tutorials.php This webarchive snapshot version has saved the pages of the mentioned resources too.
