Balsamiq: visualizing software using a mockup - quickly

What a nice story on Read Write Web today. Someone quits his job, starts writing software, and has already made $100,000 in revenues in 5 months. The idea behind Balsamiq is interesting: provide a tool that can quickly create a mock up of software or a web site. Not only useful to nail down a software specification among a group of engineers, but probably also handy when you want to communicate/visualize a very early startup idea to potential investors when you do not have a prototype or demo ready yet.
More background reading on demos: David Rose advises never, ever, to use a live demo in a pitch presentation, and an earlier post on presenting software interfaces in PowerPoint.