When I worked for the USDA as a student programmer, Steven introduced me to a rapid prototyping tool for software development. Now that I am out in the “real world” I occasionally need to mock up websites that I am developing.
In the little time I have used Balsamiq I found it easy to use and quick to get a rough sketch of what you were looking for to show the client. My only complaint is that it defaults to the crime against humanity that is MS Comic Sans. This is modifiable (kind of) to use your standard system font.
As you can see from the above video from the Balsamiq people, it is easy and quick. I could talk about it all day, however words can only do so much… they have a web launchable demo that is very usable at http://www.balsamiq.com/ .
Future plans for the product include a web app version that would enable users to not have to even install any software. Which should be fairly low hanging fruit as it is already an adobe air application (and one of the only ones that I have seen that are useful). The fact it is an air app also is a benefit as the developers are actively adding features all the time.