Brew Your Own/Winemaker Magazine
Back when we were developing sites for Joomla 1.0, there was an extremely useful little extension called PeopleBook that didn't survive the transition to Joomla 1.5. Brew Your Own and its sister site, Winemaker Magazine, provided the perfect excuse to write my own from scratch. It became so popular among my colleagues that they encouraged me to make Grumpy Contacts available for general release.
I created a number of specialized toys (er - I mean tools!) for these sites. One example: BYO needed a specialized database for managing lists of hops, grains and yeasts. Working with a Flash designer, we created a brewing calculator that uses the database for its dropdown selections.

An in-house database maintained on an AS/400, a need for the capabilities of Joomla and a little coordination with the company programmers and you have a Joomla site with a user list primarily maintained by their internal database system. How cool is that?!