In terms of databases, I can think of Monty Program http://montyprogram.com/about http://montyprogram.com/about and Percona http://www.percona.com/software/perc

In terms of databases, I can think of Monty Program http://montyprogram.com/about http://montyprogram.com/about and Percona http://www.percona.com/software/percona-server/downloads/ http://www.percona.com/software/percona-server/downloads/ who both build forks or extensions of MySQL Both seem to make most of their money through consultancy rather than development and support, but I think that's quite common in the open-source world? And, of course, you missed out the most obvious one, Red Hat, who do huge amounts of open source work. While it's now pretty old (1999), the essay "How Red Hat Software Stumbled Across a New Economic Model and Helped Improve an Industry" written by Robert Young of Red Hat was really informative to me. http://oreilly.com/openbook/opensources/book/young.html http://oreilly.com/openbook/opensources/book/young.html