The popular open source project, Growl seems to make some income. They are at number 34 in the top-grossing list on MAS. Growl is only distributed via MAS and cost $2. Previously you could download and install it for free under an open source license. According to the lead developer; "our [software repository] is [currently] locked down but once 1.3 is out, we’ll open it up." I wonder.