I have worked with several storyboarding tools (especially when I was in the games development business), the most basic of which is StoryBoard Quick from PowerProduction.
Though it is designed for video production pre-viz work, it is cheap ($170?), flexible, really easy to use, and can be applied to just about anything. It's particularly useful if you want to sketch out ideas but (like me) can't draw -- it has a large library of characters and objects, and lets you import your own images and screen shots as well.
With a little imagination you can get a lot of storyboarding done in PowerPoint or Visio, so long as you are not looking for help with creating sketches or artwork. This is fine for mapping out simple process flows from one screen to another, using text, simple connectors and cut-and-paste screen captures. With Visio everyone has to have a copy of the program in order to read the files, and that can get expensive. Most people have PowerPoint already.