The Open Souce IDE (not just for Java).
"Programmer's Text Editor"
A great O/R mapping framework.
Java Server Faces:
For everyone who has to design Web GUIs fast an consistent ...
... and here the Open Source implementation of Apache Community
The official reference implementation of Java Servlet and Java Server Pages technology. By Apache, of course.
A content management system we are using for this site - good and simple
Development in general
The Bug Killer - Very good yet simple bug tracking system in PHP/MySQL
A "soft introduction" into this programming methodology
A book on the versioning system, available online as PDF document (GNU Public Licence)
The object relational Open-Source RDBMS
GIS (Geografic Information Systems)
Powerful OpenSource-Desktop-GIS (implemented in Java)
The "user-friendly Desktop internet gis" is a GIS desktop based on the Eclipse environment
Geospatial extension of PostgreSQL
The "classic" Open Source mapserver
More than just an Open Source Mapserver (in Java) - a complete development framework
It is not open source and just available for MS Win****, but it's very powerful, easy to use and astonishingly cheap!
The Open Geospatial Consortium defines standards for GIS applications and data.