During a talk [1] at Stanford, Loren Brichter mentioned that he was living off income from Tweetie (this was before Twitter's acquisition of Atebits and the rebranding of Tweetie to be Twitter for iPhone).
He was doing a lot more than just living off it, Tweetie was the #1 Paid App for a long time and was in the Top 10 for all Paid Apps for months. He was selling thousands of copies per day for $2.99.