Eclipse SQL Client for database querying/browsing any JDBC compliant database. It supports plugins with specialized functionality for individual databases (Oracle, DB2, MySQL, PostgreSQL, Sybase, Informix) and can be extended.
Vela is a Open Source SQL and PL/SQL client software for Oracle database with a graphical user interface developed using Java Swing. It is a Front-End tool for a Oracle developer. It supports most of the common developer tasks.
SDBC is a MS Access-like database client based on Java swing ui, able to connect to both local and network DBMS. It uses Derby for file-based database and JDBC for SQl-based DBMS.
DBPad is a GUI-based SQL client for relational databases. As long as you have a JDBC driver for your DB, you can use DBPad to view or manipulate your data and metadata of your tables.