A lot of you are already using the WebDialog of the SketchUp Ruby API to extend the user interface of SketchUp. Let's say you want something with a more native-application feel to it. Thanks to Peter Ellis of NREL, there's a neat plugin out there that allows you to do this. It comes with useful GUI elements that you can use in your own plugins such as a file-open dialog box, progress bar, etc.
It's built on top of wxRuby, so your UI implementation is cross-platform and you won't have to do any extra work. Best of all, it's FREE! Hats off to Peter.