I’ve been using the free version of Balsamiq Mockups to create some application page mockups. It’s not quite as fast as pen and paper, but sure as hell beats doing it in something like Visio. I really like it*.
The application is available both online and in an Adobe Air version that can be downloaded and run from the desktop – I’ve used both and probably will continue to use both, the differences between the two versions are marginal.
Balsamiq Mockups comes in a free trial version as well as a paid for version ($79). As far as I have experienced, also the only differences between the free version and the paid version ($79) are that 1) the free version nags you every 5 minutes, and 2) you can actually save your files with the paid version, and also 3) print the mockup. To make the free version useful, Balsamiq does allow you to export the markup as XML (and re-import the xml to re-create the drawing.)
Below is a screenshot of a markup I did of mo.notono.us – took all of 10 minutes:
*… in the interest of full disclosure I fully intend to email Mariah to see if I can get myself a free license….