MySQL was unique in having a open source and commercial dual license structure.
An interview with Marten Mickos, CEO of MySQL AB.
MySQL is a very popular database that is released under a “dual license” approach that includes the GPL as well as a proprietary one. Marten discusses the differences between the two licenses and the issue of deciding which to use. He also talks about business models, ensuring ownership of copyrights in an Open Source project and in general, software patents, the fact that about 40% of MySQL’s active installations are on Windows computers, various segmentations of their market with respect to different licenses, and whether an Open Source-based ISV can make a reasonable amount of money in the eyes of investors.