"Copyleft is a general method for making a program or other work free, and requiring all modified and extended versions of the program to be free as well. The simplest way to make a program free software is to put it in the public domain, uncopyrighted. This allows people to share the program and their improvements, if they are so minded.

Recently, I collected some data from Sourceforge, hoping to find evidence for the importance of copyleft. But I found something surprising: although there's plenty of evidence that many developers believe in the power of copyleft, the one measure I could derive of how much copyleft actually works showed that copyleft made no difference whatsoever!

[...] However, it's ultimately hypocritical to claim support for a copyleft structure but oppose GPL enforcement. If you believe the license should have a legal requirement that ensures software is always distributed in software freedom, then why would you be surprised — or, even worse, angry — that a copyright holder would seek to uphold users' rights when that license is violated?

Combining freedoms and copyleft in the Gnu GPL license (invented by Richard Stallman) was the cornerstone of free software. This is now questioned due to the proliferation of incompatible copyleft licenses.

"...In the GNU Project we usually recommend people use copyleft licenses like GNU GPL, rather than permissive non-copyleft free software licenses. We don't argue harshly against the non-copyleft licenses—in fact, we occasionally recommend them in special circumstances—but the advocates of those licenses show a pattern of arguing harshly against the GPL..."

"...copyleft is a tool designed to spread freedom in software, at any cost. The FSF, which wrote the GNU and Affero licenses, has the goal to make all software free as in freedom. Companies are ‘free’ to join the revolution, or to look somewhere else for their needs.

Terry Hancock made a few statements about why developers choose a copyleft licence as did Tony Mobily in his editorial for issue 20. So let me tell you why this developer chose (and continues to choose) a copyleft licence?

"FSF's John Sullivan will talk about copyleft, copyright, the War on Sharing, and the GNU/Linux free software operating system at the Connecticut Film Festival." -- via Copyleft and the War on Sharing - Free Software Foundation

"My LCA talk on Friday was about why open video is critically important to free software, and what Mozilla is doing about [...] So why doesn't Mozilla just license H.264 (like everybody else)? One big reason is that that would violate principles of free software that we strongly believe in.