But, what about Cocoa-Based widgets? :?:
From Zack Rusin's blog:
I finally got most the implementation of the HTML Canvas element for KHTML finished. It's in the kdelibs-js branch in SVN. After George/Maks merge their other changes we'll merge it to HEAD. I'm planning to add full OSX Dashboard compatibility layer for Plasma (hence why I've spent most of the day yesterday on implementing the Canvas element)...
In that post, there are other interesting things like a "desktop layer" in KDE4 that make available some amazing effects, like a "water-like surface" over the desktop; or some usage of XGL accelleration.