I frequently want to quickly 'mockup' a sample page, dialog, or even application. Typically, I start with a pencil and pencil. However, with teams distributed around the country, and world, pencil and paper mockups are just too difficult to share.
I've tried Visio, paintbrush, bamboo pen pads, taking pictures of my mockups, and snagit. However, no matter how much I try other tools, I keep coming back to paper.
Balsamiq Mockups has the potential to offer the best of both worlds. It's a very clean, easy to use, flex/air application that enables very quick mockups. Typically, in a mockup, I'm striving for placement, layout,and broad concepts. This is where Balsamiq shines. The images are 'simple' enough to leave enough room for imagination.