A few companies have used dual-licensing with at least a certain degree of success - you either use their software under the GPL, which means any software you c

A few companies have used dual-licensing with at least a certain degree of success - you either use their software under the GPL, which means any software you create using it has to be GPL, too, or you can buy a license that does not force you to use any particular license. Sleepycat did it with BerkeleyDB (they were bought by Oracle, though, and I don't know if Oracle changed this arrangement), the same goes for MySQL (which, of course, is now owned by Oracle, too). There are a few more, I guess, that I do not know about. OPSI is backed by a company and uses a model where OPSI itself is free software (in both ways), but new features are made available to paying customers only initially, until development of these features has been paid for. Also, there are cases of companies making software open source, but making money by doing one-off customization or (obviously) support contracts.