Of late I’ve had to use Apache Derby in my work.
SQuirrel SQL is a great, Java-based client for connecting to any JDBC compliant database.
I recently did a couple of screencasts (at my place of work) on installing and setting up SQuirrel SQL to work with Derby.
Here are the links to the screencasts:
Installing SQuirrel SQL Client for Apache Derby
Set up SQuirrel SQL for Apache Derby Database