My understanding is that that is the update center for the "contrib" module. Or, that the "contrib" module is included, together with other update centers. Either way, the "contrib" module is included. (That's the module containing community contributed modules, often in "alpha" state, so caveat emptor.) You can then... install modules called "DocBook Projects" and "Docbook XML". Then you have a new project type (as well as templates and other features, such as HTML generation for a DocBook project and a cool image insertion feature, via a related module that is automatically installed) for DocBook (as used when writing the "Rich Client Programming: Plugging into the NetBeans Platform" book).
Another one that I found that I like is called "Project Minder":
It didn't work for me when I installed it in NetBeans IDE 6.0. But when I installed in a development build, I could use the keystroke described above, which made the editor tabs flash for documents that belong to the same project as the current document. That can be quite handy.
And if you install a module called "Extra Update Centers", you'll find yourself with many update centers, all containing modules worth exploring:
In the list above, deadlock.netbeans.org is the update center that I referred to at the start of this blog entry, so if you just install "Extra Update Centers", you'll get this one too.