TheQwerty: looks like something that might be worth giving a go - I feel pretty weird about using FireFox for non-web stuff, though . Oh, and it looks like the mockups look like native GUI widgets?
It is weird using it within Firefox. I just revisted their official website
and there is also an XULRunner build available that might have fewer quirks; the Firefox add-on doesn't handle resizing the work area too well.
They aren't really native, it's basically all just image overlays using different collections. By default it came with collections for Win XP and GTK but it's supposed to be extendable. The Google Projects
page appears to have downloads for ExtJS as well.
It fit my need for something
free to mock some small single dialog apps. The other products listed look much better, but there's no way I can justify spending anywhere near those prices for something I'd hardly use.