To underscore Patrick's point about just how many of these businesses there really are, he's one of the few folks I remember from the old Business of Software forums over at joelonsoftware.com that seem to have migrated over here.
That was a tiny little board where the owners of single player software businesses would get together to ask silly questions about how to update their shareware product's PAD files. There were a few hundred of these businesses represented there just among the regular users.
Most of them fly under the radar since, having products that provide a comfortable living with relatively low workload and no need to follow the latest hot technology trends, there's little reason your average Dot-Com-Thousandaire would ever stumble across a place like Hacker News.
Edit: forgot to plug S3stat (http://s3stat.com) and Twiddla (http://twiddla.com), my two main revenue generators, and FairTutor (http://fairtutor.com), hopefully the next one.