*** UPDATE ***
The instructions below are still valid for Netbeans 6.9, but there is a new repository link for Netbeans 7.x, have a look here for more.
In the latest Netbeans releases, for some reason I can't understand, they removed the XML and WSDL plugin from the list of default available. This plugin is one of the best tools you have in Netbeans, as it allows to create and modify XML files, XML Schemas and WSDL files easily and graphically. It is a reduced version of what you get with Altova's XML Spy.
Browsing the Internet I have found a Netbeans repository from which I can install this plugin for Netbeans 6.9 also, what I did is to add a new Plugin download URL:
Then I added the new plugin center:
From where I can install some development plugins:
To know what it's possible with this plugin, have a look at: http://xml.netbeans.org/
I'd like to now if the URL above is the right one or there are different places from where it is possible to download updated releases of this.