If you are responsible for designing application workflows and haven't heard about balsamiq, go there NOW and download the demo. Don't even bother reading to the end of this paragraph. Just go.
I've been using this program now for a few months and it has been fantastic in getting those pesky requirements down on paper before you spend a ton of time doing markup and css.
It takes a little practice to get quick with it, but you'll be amazed at how effortless the interface really is. The biggest problem for me originally was deciding on a simple drawing storage plan. I basically wound up with a simple "one folder per project" model and that has worked nicely.
One of the features I really like is the ability to pull in graphics from existing sites for maintenance work. The best feature about this is that balsamiq will let you degrade the real image to make it look more like a quick sketch. So your initial design goes from this:
At first, this might not seem like a big deal, but it winds up being huge. In the minds of your clients, that black and white sketch is just a sketch. That color image on the other hand is almost finished and they don't understand why you are dragging your feet so much considering that you've already shown almost the whole application.
There is a linking option in Balsamiq that will allow you to simulate linking your projects together. It makes for a nice flow in a live presentation, but I'd urge caution in it's use. Again, it looks a little too polished. Having to open a separate file for each page transition makes it obvious that what you are looking at is a series of pictures and not markup. Once you get your client trained on what they are looking at, then maybe you can make good use of the linking.
Another plus is that Balsamiq is an AIR app. At this point, it's my favorite AIR app. It's always nice when something you use is written in the products you work with.
So, if you do any type of application design work, take a few minutes and check out Balsamiq.