Here are some open source database tools that caught my eye. I haven't had much of a chance to try them out, but I'm glad they exist, and I hope one day I get the chance to use them...

Talend
http://www.talend.com/index.php
Data migration tool. Extract/Transform/Load (ETL). A collection of Talend Java classes that Load, Transform and Store data. Talend has a IDE based on eclipse where you can visually wire these Java classes together. The GUI just generates a Java file which instantiates and configures the Talend Java classes, so you can edit the code outside of the IDE, and extend the Talend base classes with custom behavior.

Pentaho
http://www.pentaho.com/
Collection of tools for ETL, Reporting, On-Line Analytical Processing (OLAP), etc.

JasperSoft
http://www.jaspersoft.com/
Reporting tools... I think everyone's heard of this.

LiquiBase
http://www.liquibase.org/
Java database refactoring framework which lets you rename tables, columns, diff database structure, etc). Also has hibernate tools to compare hbm.xml files to a schema structure, so that you refactor your hibernate POJOs and refactor your database at the same time! Wohoo!

DB Unit
http://sourceforge.net/projects/dbunit/
Extension of JUnit aimed at writing tests for your database, so you can make assertions about your database environment.