Custom support, service, and software improvement contracts

are the most widely used models for GPL'd software. The GPL is

central to their success, because it ensures that the code base

remains common, and that large and small companies are on equal

footing for access to the technology. Consider, for example, the GNU

Compiler Collection (GCC). Cygnus Solutions, a company started in the

early 1990s, was able to grow steadily simply by providing services

for GCC --- mostly consisting of new ports of GCC to different or new,

embedded targets. Eventually, Cygnus was so successful that

it was purchased by Red Hat where it remains a profitable division.

However, there are very small companies that compete in

improve it for everyone. Meanwhile, companies who rely on GCC for their

work are happy to pay for improvements, and for ports to new target

platforms. Nearly all the changes fold back into the standard

versions, and those forks that exist remain freely available.

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\label{Proprietary Relicensing}

A final common business model that is perhaps the most controversial is

proprietary relicensing of a GPL'd code base. This is only an option for

software in which a particular entity holds exclusive rights to