Java

• Navicat - currently used by Paul Burney, Dima, Trudi and others, it's a cross platform application that is MySQL specific. It's an admin tool, not a design tool. Has a nice graphical query builder but it does inner joins rather than outer joins so using it to generate efficient code for embedding in programs is questionable. Good import and export tools.
• SQuirreL - LGPL it has nice classpath support for loading in multiple jdbc drivers. It works with older versions of hsqldb. It shows a tree view of the tables and it is easy to see their content. It doesn't allow you to edit tables, without writing sql. It doesn't have a gui query builder. It doesn't have a gui form builder.
• Fedy's SQL Admin
• ViennaSQL
• Servoy

Eclipse Plugin

• eva/3 - is a full rapid application environment, one part of this is a gui database admin tool. It has a free non-comerical license. No source code. The latest version doesn't seem to work with older versions of hsqldb.
• SQLExplorer - LGPL this is an eclipse version of squirrelsql. It seems to have exactly the same features. But it looks a lot nicer in eclipse.

Gnome

• gASQL
• gmyClient - looks kind of dead. but looks nice