Java Add-On Development Kits?

Wombchimp asks: "For all of us who program in Java, the development kit is vital to do the slightest piece of coding. But I seriously doubt that there is one person who has not made some custom utility class to fill a void in the JDK. Why is there no real widely known organisation to collate and develop an add-on development kit which could fill this void and collect a database of useful utility classes? Most sites seem to offer full blown applications, but the odds are it took some useful assistant classes to build it. The closest I've managed to find is the Giant Java Tree, which isn't really organised enough to see if what you want has already been coded, and Gnu's Java Library, which doesn't really have enough. What do you think? Is there some site already filling this gap? Or is this idea a waste of time? "