Copyleft is a play on the word Copyright to describe the practice of using copyright law to offer the right to distribute copies and modified versions of a work and requiring that the same rights be preserved in modified versions of the work. In other words, copyleft is a general method for making a work (text, music, computer programs, Images, etc....) free and requiring all modified and extended versions of the program to be free as well.
Copyleft is a form of licensing and can be used to maintain copyright conditions for works such as computer software,documents, music and much more. In general, copyright law is used by an author to prohibit others from reproducing, adapting, or distributing copies of the author's work. In contrast, an author may give every person who receives a copy of a work permission to reproduce, adapt or distribute it and require that any resulting copies or adaptations are also bound by the same licensing agreement.[From Wikipedia]
The websites listed here are starting places for your search of Copyleft materials on the web. Other websites may also be helpful, just be sure you know how the original creator intended for their work to be used.