Marshall Kirkpatrick blogs about how Peldi Gullizzoni made over $100K selling Balsamiq, a UI prototyping tool he built himself. It took me a while to see what was so special about Balsamiq. It wasn't the first tool to address that problem, nor the most full featured one. At $79 a pop it was reasonably priced, but not exactly cheap. The thing that does stand out about it is the use of hand-drawn visuals rather than pixel-perfect ones. This gives it a unique, "designer" feel and a fun-factor that sets it apart and lends it a special character.
Related to my earlier post on products that improve life, I should add the sub-category of products that entertain us while solving our problems. That extra entertainment factor is easily overlooked or dismissed, but it is precisely the extra character in otherwise boring products that makes all the difference in the market.