Until a year a two ago, mySQL seemed to be the open source database to back, but barely a week goes by without SQLite popping up behind the scenes of some program I use - it powers both Aperture and Lightroom. Today's Guardian carries an interview with SQLite's creator:

It's very clear to me that if I'd had any business sense whatsoever I could have made a lot of money, but I don't. I like to tell people that we make enough to live fine in Charlotte, North Carolina. We don't make nearly enough money to live in London or San Francisco, but we don't live there so that's OK.