You will not dispute that modern desktop application needs some theming, right? And, being Swing developer, few times you tried to solve this problem, didn't you?..
I have solution now: JLAFChooser.
Now, you can just put JLAFChooser component into your setting dialog, and it will make LookAndFeel chosing possible in runtime. Now you have no need to hardcode all the LookAndFeels you know into your classes or XML files, and user can just download few LookAndFeels and put it somewhere in classpath to make it available to JVM.
It has two main variants: when Apply is allowed and when not.
When Apply is allowed your user can choose something from list and check it at once, and may be choose something else.
When Apply IS NOT allowed you can just show list of possible LookAndFeels, and save somewhere to preferences user selection, to apply it next startup.
Usage is similar to JColorChooser, with few extensions - you can get LookAndFeel instance, it's name or it's class name. Methods to apply and restore LookAndFeel are public, you may use them for your controls.
PS: feel free to mail me for suggestions, bugs, or just to talk about Java, Swing, religion, fantasy or anything...