I saw Tim's post on Software Mockups using Balsamiq just a few days after independently 'discovering' it via Russ.
I'm a big fan of 'visuals', so I had to immediately give it a try, mocking up some additions to Geoquery 2008's user interface:
I think a 'google earth globe' is about the only widget they didn't include!
Balsamiq Mockups is great fun - a very wide variety of widgets are built in, the Flash interface is easy to use (and if you can't get stuff to quite line up exactly, who cares! it's a mockup!). If you continue to use it, don't forget to buy.