The Balsamiq Mockups guy has made $100K in 5 months. I Faved about Balsamiq a little while back: http://faves.com/users/mohit/dot/127498885101

Quoted: That was a cute story, but now it's been just 5 months and today Peldi reports that he's just cleared $100,000 in sales of the four variations of his product. Talk about a simple tool coming along at just the write time! It's cool software, too.