Depending on what you are talking about there are different answers to that question. WikiPedia is funded by the charitable non-profit WikiMedia Foundation. Other companies, such as SugarCRM, release a subset of their full product as open source with additional features present in their commercial editions. MySQL dual licenses their products charging for technical support and special licenses for companies needing to integrate the MySQL software into their proprietary software systems.
Many open source products don't make any money at all but are merely maintained by volunteers or the generous support of corporate sponsors. There are so many different ways that open source is used and developed in the business world that there is no single answer to this question.
