GPL (the spider plant license)
The GNU Public Licence was one of the early Open Source Software licenses in wide use. It was popular in the Linux development effort. At times it has suffered from some open-source and free software community dichotomy that has been partly resolved with the third version. Commercialisation issues have hampered some of the growth potential and incompatibility with a few of the other popular open licenses and GPL limits the integration of useful code bases between some popular projects.
Details can be found on the Wikipedia GNU page and makes for gripping reading. The official GNU Project site has the formal text and other details for the latest (GPLv3 in 2016 at least) and earlier versions.
Adrian Bowyer wrote early on his thoughts on why he believed the RepRap project would work best with Open Source disclosure and why he was able to accept others who would be more restrictive in terms of licensing, believing one supposes that the best system would win out in the end when allowed to evolve.
He also documented his choice of GPLv2 or later for the RepRap project and documented the RepRap GPL licence to cover those parts that could be copyrighted and how they should be protected. This should be read in conjunction with the official GNU pages.
You can generate a list of local wiki pages that link to this page. It will give an indication of projects that are published under GPL licence. Errors and omissions may exist so be careful and inspect the page contents before making choices.