I'm looking for a web app that would allow me to quickly sketch some prototype screens for a potential web app for users of this application. Is there a preferred tool for this kind of thing? Or, if not a web app, is there offline software for doing this? Should I just use something like Illustrator/Fireworks?
I've used Excel a few times, you can create a grid of small squares, then colour in whatever you need and add all of the basic controls.
I quite like Omnigraffle on the Mac. It's very versatile and there are many stencils out there to get you going.
At work we use Axure. It works well. It allows for exporting the prototype to HTML and you can even add small interactions if you want. It works better then Visio in my opinion since it's specifically made for the this purpose.
these type of mockups are also called a wireframe which might be a helpful term if you want to search for more information.
Fireworks has always been my friend being that I can create so many layers of the evolution of the UI in a single document. I haven't upgraded to CS4, but I heard you can make interactive prototypes in it.
For an alternative to balsamiq, check mockingbird!
Mockingbird is an online tool that makes it easy for you to create, link together, preview, and share mockups of your website or application.