Two projects that lived and died before their time
WebOnSwing WebOnSwing is a project that allows you to migrate Swing applications to a web environment with few modifications. It would be good for Groovy because the Swing(X)Builder could target desktop as well as web applications freeing us from the stranglehold of HTML and CSS. But alas, it wasn't meant to be. WebOnSwing is dependent mostly on Java 1.3 - 1.4 with a buggy patch for 1.5 and no release plan for 1.6 or higher.
JD4X JD4X is a window manager for Linux implemented in Java. Back in the day, the idea was probably laughed at. With the upcoming release of Update N, this is seeming like more and more of a good idea. Preloading classes for faster app startup, shared or multiple JVMs, and Groovy UIs on top of that...it would be awesome. The good thing is that if someone decided to take up the task, there is ample documentation and the system is built with 1.4 - 1.5 in mind.