I have been looking for a tool to build back-office applications within Joomla! websites and recently found Fabrik. I absolutely love this extension. It installed in Joomla! 1.5.15 with no problems at all and I had a prototype of a customer service portal (customers log in to see the status of their account) working in about five days. This was using only the free download and the publicly available docs and tutorials. If you know relational database design you will find Fabrik easy to understand.
I plan to subscribe to get the more advanced plugins and access to the advanced tutorials. Fabrik appears to be the tool of choice for anyone who needs to build vertical market solutions without coding.