Open Source Licensing – Is It Fair to Require Copyright Assignments?

I'm trying to make this answer as 'meta' applicable as possible.

Using snippets / bits from other projects

Clearly mark the code with the original author's copyright. Make sure that your license of choice is fully compatible with the license of the code you are using. You will need permission of the author to move the code to a different license (unless they specifically allow you to do so, I.e. "GPL 2 or any later version")

Your program should have an AUTHORS file (or similar), where you list all contributors and things that you used from other projects.

Forking a project

For each module that you substantially change, add your copyright under the original author's. The same thing goes for licensing, you are bound by the terms of the license that was in effect when you forked it. If the project says "GPL2 only", you must respect that, you can't go to GPL3 without their permission.

This varies, greatly, depending on the license at hand. The QPL says you can only distribute changes in patch format, for instance, so make sure you understand the terms that allow you to distribute modified versions of the software.

Beyond that, always preserve copyright. If adding your own copyright to what exists, be sure that you clearly mark exactly what you are claiming.

A lot of your confusion will go away if you keep in mind that copyright and licensing are two completely separate things.

Copyright is owned by the original author. (Unless it was produced under a "work for hire" contract, in which case it is owned by the employer. This most likely doesn't apply to your project.) Copyright can be sold or otherwise transferred.

Licensing is the copyright owner giving other people the permission to use their work in specific ways, under specific conditions. Distribution of a work under a license does not transfer copyright rights. (Unless the license specifies that it does, of course, but no open-source license I know of does this.)

This has two important implications for open-source projects:

• If you license your own code, that you hold the copyright to, under an open-source license, there is nothing stopping you from licensing it to someone else under completely different terms, because you still hold the copyright.
• If you are running an open-source project with multiple contributors, you do not own the copyright to work that other people have submitted, unless they explicitly transferred copyright to you. Since you don't own the copyright on the entire project, you do not have the right to distribute the entire project under an alternate license unless you've made sure to get permission to do so from every contributor.

If you're worried about this, the best way to take care of this is to make sure that every contributor agrees to allow you to manage licensing before you merge any of their contributions into the main project.