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