If you've used, or are using HSQLDB (http://hsqldb.sourceforge.net/) I'd appreciate a brief description of what you did with it and what you think. I'm trying to get an idea of how much usage this software actually gets.
We're using it in the automated integration tests for the Spring/Hibernate/Oracle project we've been working on for the last nine months. Can't say anything about its behaviour in a production situation, but it is happy enough to handle the sometimes hefty SQL statements Hibernate throws at it during the tests.
I used it to deliver an Access like Swing Application for someone that wanted to persist all his Star Wars crap. He had one in MS Access so I basically mirrored its functionallity and used HSQLDB so he didn't need anything extra to install.
We have previously used HSQLDB as a server for the very-early startup phases of projects, knowing that it would be replaced before the project got too serious. It's a bit hard to explain the decision process, it was a while ago. We tend to default to MySql now when the database is unspecified.
I also tend to plug a standalone instance into web apps when I'm just scratching out a thought.