Many open source projects have long become commercial. This paper shows just how much of open source software development is paid work and how much has remained volunteer work. Using a conservative approach, we find that about 50% of all open source software development has been paid work for many years now and that many small projects are fully paid for by companies. However, we also find that any non-trivial project balances the amount of paid developer with volunteer work, and we suggest that the ratio of volunteer to paid work can serve as an indicator for the health of open source projects and aid the management of the respective communities. Index Terms-Open source software, empirical software engineering, volunteer open source, paid open source.