I'm a fan of swing and find it has done everything I need except for extensible GUIs. I can make small parts extensible programmatically, but if I want to plug in an optional feature it can be quite tedious to dynamically generate every component being altered.
The short period I spent writing a Firefox plug in got me very interested in the idea of Overlays.
Does there exist anything for Java with a similar concept?
There have been multiple attempts at projects that can convert an xml document into a java GUI.
JavaFX would be ideal, but that requires 1.6u10 or better.
It would of course be good to know more precisely what do you mean by "extensible" GUI.
In my work most needs for extending Swing components are nicely fulfilled by JXLayer. For example, things like implementing validation notification layer, blocking with progress indication etc.
"to plug in"
Do you mean plugins?
Take a look here: http://karussell.wordpress.com/2009/09/16/plugable-swing-a-hello-world-osgi-example/
similar things are possible with NetBeans lookup class (openide) and the ServiceLoader included in JDK6: http://weblogs.java.net/blog/2008/08/12/simple-dependency-injection-serviceloader-jdk-6
For an extensible GUI framework look here: http://karussell.wordpress.com/2009/10/08/java-application-frameworks-not-only-client-side/
Update: Now with griffon you can do this too
Take a look at the SwingBean framework.
It's kind of cool, because it allows you to generate user interfaces through xml files.
Here is the link: http://swingbean.sourceforge.net/