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.
Cool tool...http://www.balsamiq.com