All plugins hosted in the WordPress.org repository must be compatible with GPLv2 or later. That means all code that is on our servers, from images to CSS to JS to the PHP code, has to meet that requirement. This is an extra requirement than just the standard one of derivative code, but we strongly feel that proprietary content has no need to be in our repository. If your code needs to be split licensed, or you have to included proprietary code for any reason, we can’t host you on .org, but that has no bearing on how neat and cool your code might be.
For a list of various licenses, and their compatibility with GPLv2 please read this: http://www.gnu.org/licenses/license-list.html – We know not all of you are lawyers, and thankfully that list makes it easy to check what licenses do and don’t mesh. If something doesn’t have a license, ask the author please, and don’t assume.
The following code bases are popular (which is to say we see submissions with them pretty regularly), but at the time of this post, are not licensed GPL-compatible. None of this means you can’t or shouldn’t use this code on your sites or plugins, just that we can’t host it here if you do.
If there are plugins you find using these (or any non-GPL-Compatible) code bases in their plugin, please email plugins AT wordpress.org and we’ll get in touch with the developer. If you’re the author of one of those code bases, please consider re-releasing your code under a GPLv2 Compatible license! We’d love to be able to host your work here.