If you haven't already taken a look at Greasemonkey, you really ought to. Maybe even right now (note: Firefox required).
Greasemonkey is a Firefox extension that enables folks to modify the contents of a website in realtime. For example, if your favorite blog's comment textarea is too small, make it larger yourself. Forced to manually click a bunch of checkboxes? Automate it. A bunch of Greasemonkey scripts are available to fix general website problems, and a bunch more to add new functionality to popular websites.
This is huge. We've had to put up with poor user experiences in applications ever since the dawn of the desktop computer (some more than others). Being able to fix annoying website UI glitches is a wonderful experience.
But I want more. Some of my most frustrating experiences involve desktop applications (such as a certain personal finance software package); what's the equivalent of Greasemonkey for such apps? More relevant to this discussion, what's the equivalent for Java desktop apps? Could it be something like exposing the Swing component hierarchy to power users for manipulation through some sort of general tool?
Yikes. The web's innate separation of client and server tiers makes this kind of thing seem much safer to me. You can't use Greasemonkey to manipulate moving parts that aren't exposed to the UI tier; having somehow directly interface with the business logic tier of a desktop application scares me to no end.
If not a general Java state manipulator (a la modern debuggers), is it external scripting against a published API, like AppleScript or ActiveX? The general problem with this approach is that these APIs are often too coarse-grained to enable the type of powerful modifications that Greasemonkey make's simple. Automator on OS X is nice, but its not nearly powerful enough for this kind of interaction.
Regardless of the specific implementation, the notion of empowering users to customize the UI in ways the designer never intended seems so compelling as to be a feit accompli in the future of application development.