Pencil is an animation/drawing software for Mac OS X, Windows, and Linux. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics. Pencil is free and open source.

"So here is the deal: When you develop for a market, you don't go by the user base. You go by the potential customer base. That's what most software companies do. They base what they want to create on the size of the market they're developing for," Warde

Sitemappers.com lets you easily whip up a prototype sitemap to help your clients visualize their site structure.