The Linux kernel is an enormous open source project that has been in development for more than 25 years. While many people tend to think of open source projects as being developed by passionate volunteers, the Linux kernel is mostly developed by people who are paid by their employers to contribute. According to The Linux Foundation, since 2005, “some 14,000 individual developers from over 1,300 different companies have contributed to the kernel.”
About once a year, The Linux Foundation releases the Linux Kernel Development report with data about release frequency, rate of change, who contributes, and which companies sponsor this work among other things. The 25th Anniversary Edition released in August 2016, covers development through the 4.7 release (July 24, 2016), with an emphasis on 3.19 to 4.7, which were released since the previous report in February 2015.
One of the most interesting data points is the decline of contributions from unpaid developers, which has decreased to just 7.7 percent in the period covered in the 2016 report compared to 14.6 percent in the 2012 version. With Linux kernel developers in such high demand, many of these unpaid developers have been hired by companies who employ people to contribute.