And those too: (Score 1)

Some usefull stuff that I actually *use* besides all the great tools mentioned above (in no particular order):

• http://ostermiller.org/utils/com.Ostermiller.util Java Utilities - nice CSV tools among others
• http://protomatter.sourceforge.net/Protomatter - misc tools - object pools etc. (I know, I am lazy)
• http://jedit.org/ JEdit - lightweight, with tons of plugins highly customizable Java IDE/Editor (for those who find the great Eclipse too heavy)
• http://jetty.mortbay.org/jetty/ Great 100% Java Web Server / Servlet /JSP Container with JMX etc. etc.
• http://sourceforge.net/projects/quartzEnterprise Job Scheduler
• http://mandarax.sourceforge.net/Great open source java class library for deduction rules
• http://alphaworks.ibm.com/Alphaworks as a whole are full of java goodies
• http://eclipse.org/aspectj/Did you ***** up your design ? Your project is a zombie that should be trashed and rewritten ages ago ? Aspects will save your day ;)