Affero General Public License
Eh, I am not so well informed. GNU AGPLv3 was published in late 2007 and, finally, it may be used without thinking about compatibility with GPL. MediaWiki and other Wikimedia software (as they deal with Web) should be licensed under this license. And, of course, I’ll use this license for my software.
Here is the quote from the Software Freedom Law Center‘s document “A Legal Issues Primer for Open Source and Free Software Projects” (thanks to Slashdot for publishing an article about this document):
The GNU Affero General Public License, version 3 (AGPL), is a variant of GPLv3 published by the FSF in late 2007.6 The AGPL contains an additional requirement regarding network deployments of modified versions.
In general, the GPL’s source code provision requirements apply only when software is directly distributed in object code form to a user. If you do not distribute the software, you have no obligation to distribute source code. Therefore, if people interact with your software in a way that does not require them to have a copy of it, most of the GPL’s source-code provision requirement’s do not apply
The most common example of such interactions is online or networked applications in a client/server environment. For example, when a browser sends a request to a website to perform an operation (such as database entry that occurs by way of a CGI script), the web server performs the operation on the client’s behalf. The client does not require a copy of the software to perform the operation.7. The GPL imposes no requirement to provide users who visit your website with a copy of the software that runs on your server, even if your server software is GPL-licensed software that you have modified.
In a lot of cases, this makes sense. Servers and clients interact in a lot of circumstances, and offering source code to every client that interacts with every server would be cumbersome. In many instances, it would be impossible.
In some cases, though, offering source code would be trivial, and yet significant improvements are made to server-side software that never get contributed back to the community. For example, because the GPL does not require it, people rarely make the code behind their websites available to users of the site. It is in these cases that the Affero GPL can help.
The AGPL requires that you offer corresponding source code to users who interact with your modified version of AGPL-licensed software over a network. Therefore, if you build a server based on improvements to AGPL-licensed software, those improvements must be made available to all who use the software via the network.
Some copyleft advocates regard the AGPL as the next logical step toward software freedom. As more and more software is delivered as a service instead of as copies of software packages, the Affero GPL can ensure that freedom endures even as distribution dwindles.
The Affero GPL is not right for everybody. Some communities do not want the added burden of packaging their code for release to network clients. The Affero GPL creates code distribution requirements for a class of people the FOSS world has traditionally treated more as end-users than developers. Even though some end-users are now sophisticated enough to customize their installations of open source server software, your project might not want to require them to publish those custom changes. Moreover, there are many individual developers as well as corporate commercial users who believe that AGPL takes the idea of copyleft too far.
If you think the Affero GPL is the license for your project, visit the FSF website for a copy of the license and some instructions on how to use it.