A comment about "public domain" works might be in order. It is possible for something in the public domain to be taken, modified, and copyrighted or patented. There is no license protecting it from such treatment. There have been cases of people placing their works into the public domain and then losing the right to publish them because of this. I think placing software in the public domain is a mistake. Someone might grab it, and there would be no recourse.
The GPL is viral. Its intent is to "infect" derivative works to prevent someone from trying to grab the rights to use and distribute the software (or other product). The intent is to keep the software free as in speech. There is no sinister plot, no minions of the Dark Side trying to subvert Capitalism. Just some people who feel that everyone should have access to computers and software, and who are willing to create that software and ensure it's not misused. They're not hijacking or "pirating" anyone's software; they're simply offering something else instead.
UPDATE: OK, the word 'viral' was horribly chosen, and I repent of using it. I shall pound my head with a brick, as punishment.