includes SQL editor with syntax highlighting, code completion and templates, visual editing of database objects (tables, indexes, etc.), debugger of stored procedures, editable results, import/export features
DB Copy Plugin is a plugin for the SQuirreL SQL Client (1.2beta6 and 2.0 RC1+) that allows copying database objects (schema def and data) from one session window to another. The sessions can be disparate database vendors (Oracle -> MySQL, for instance).
Lightweight SQL querying tool written in Java. Connects to any JDBC compliant database and will run on any OS with a Java VM. Features: keyword highlighting, result formatting, statement history, multi-window, multiple database connects at one time.
SQLMinus - "sqlplus minus the pain". This SQL Client (Java) is very configurable, completes table and column names upon tabbing. Completes other words and abbrevs (vi-style). Powerful auto-linking across tables. Powerful auto-joining across tables.
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.
JSQL Edit is a database-independent SQL client that allows graphical manipulation for making queries to databases. JSQL Edit is written in Java and uses JDBC for connectivity and Java Swing for a graphical user interface.
ProDBA The cross platform Oracle development/administration tool written in Java. Features a powerful SQL editor and schema navigator. Can export dynamic queries to the web.
ViennaSQL is a GUI SQL client written in 100% Java. ViennaSQL can communicate with any database that has a JDBC driver. It should run anywhere Java runs.