I stumbled across what I think is a great software tool for quickly creating mockups, or wireframes, of web pages, software, or anything else you can think of. It really recreates the whiteboard / sketchpad feel in a piece of software, and is just as fast and easy to use. It's created by Peldi Guilizzoni, and he has followed through on his promise of being completely transparent with his startup and disclosing revenue, profits, etc.

Founded in March 2008, Balsamiq Studios creates rich, elegant, high quality plugins for Web Office applications. Our first product, Balsamiq Mockups, netted over $100,000 in less than 5 months and is gathering rave reviews.

It comes in several versions, including desktop, Jira, and xWiki integration. I am just using the desktop version, which runs on the Adobe Air platform.  The screenshot below gives you a feel for the interface.