Shout out to Balsamiq for believing me when I said I had a license but I changed laptops. They sent me a new license (thanks, Val).
Check out their product Balsamiq Mockups. It's a low-fidelity wireframing tool that has an informal hand-drawn look. I have used high fidelity wireframe tools and one of the problems is that people fall too easily into pixel pushing if it looks like it's meant to be a final product.
At any rate, check them out; it's $79 and way cool--and they're way cool!