I have been evaluating Balsamiq Mockups this week to see what it is all about and to determine if I want to try to get it at work.
Balsamiq Mockups is a tool (built with Flex and Adobe® AIR) used to mock-up site designs and layouts. It has a really easy-to-use, drag-and-drop interface with a LOT of customization options. It has a huge variety of web site elements for you to drop into your mock-ups. In about 15 minutes I was able to put together this: