Update from the Plugin Review Team: This post is no longer being updated, so please use the guidelines and FAQ for any information you’re looking for.
Caveat: While this topic always brings up people arguing that the GPL means they can (and yes, you can copy GPL plugins and do whatever you want with them), we wish to remind developers that just because the GPL allows something doesn’t mean we will host it here. Our guidelines are considered above and beyond the GPL. After all, the GPL doesn’t say you can’t punch someone, but if you get into a fistfight at a WordCamp, we’re not going to host your plugins.
Taking someone’s pay-for code and re-releasing it as free-of-charge is considered (by us — the Plugin Review Team) to be a form of piracy and is not welcome here. It doesn’t matter if the code is GPL, it matters that When you do that, when you copy and re-release someone’s code without any changes, you’re stealing the opportunity of the original developers to make a living, and we feel that is detrimental to the community. In addition, it’s often in violation of the terms you agreed to when you downloaded the plugin from the developer in the first place.
By you doing that, and rehosting here, you put the entire directory in peril. Arguably we become responsible for your actions. As such, we do not permit plugins that are sold off WordPress.org to be re-hosted here.
The only exception to this (besides it being your own plugin) is if you have made a significant fork, properly credited in the readme and inline code, and everything was 100% GPL compatible, including the terms from where you bought the plugin. If you pirated a plugin, or if you violated the license purchasing terms (which may say things like you cannot resell it), then we cannot host the code.
Edit: It’s important to note that adding non-GPL compliant terms to a license may in fact invalidate the license, which means we can’t host it here anyway. The above comment is not in support of people violating licenses nor are we attempting to protect and help those people in any way. We are trying to point out that even if a license says it’s GPL, if it’s sold with terms that violate the GPL, it cannot be hosted here either. tl;dr? If the license or terms are sus, we can’t host it.
If the plugin is your own plugin and you just want to re-host here, we will do our best to validate that claim, and may pend your plugin while this is researched. We appreciate your patience when that happens.
If you feel someone took your plugin and hosted a copy of it here, please email [email protected] with a link to the plugin as it’s hosted here, a link to your original plugin, and (if the plugin is hosted outside WordPress.org) attach a zip of the plugin so that we may compare the two.
Edited to add: This post is not about the GPL. This is only about the existing WordPress.org Plugin Developer Guidelines. You should not, under any circumstances, use this post to frame your understanding or interpretation of the GPL as it is not intended as such. Again, this post is about the plugin guidelines, the ones all plugin devs already committed to following, which have long since stated that immoral or ethical practices are not permitted here.