The first example of dual-licensing was probably Ghostscript, which Peter Deutsch licensed first under the GPL and later under the Aladdin Free Public License, but also under a proprietary license.
Inspired by his idea, David Axmark and I released MySQL under similar dual-licensing terms. Dual licensing has since become one of the most common and popular ways to create profit centers around Open Source/Free Software, in addition to support and services around the product.
To be able to bootstrap MySQL Ab, we originally had a license that allowed free usage, but a "pay-for" license if you used MYSQL for commercial usage or on the Windows platform. In 2000 we changed the free license to GPL, mostly to avoid having to explain our own license to everyone.
The basic idea for our dual-licensing was this: if you bought a …