Some successful open-source companies are using a dual-licensing scheme. In short, software is released both through an open source version (e.g. licensed under the GNU GPL) and through a proprietary license usually reserved to paying customers.

Recently word started getting around that the terms for getting apps on Windows Phone 7, and indie games on the Xbox, have changed. Now, programs submitted to Microsoft cannot have any code licensed under a copyleft license. Even if a single file is licensed under a weaker copyleft license like the LGPL, Microsoft will apparently reject it.

Openfire formerly known as Wildfire is a popular jabber/XMPP server written in Java and dual-licensed under both a proprietary license and the GNU General Public License. Openfire is easy to setup and administer, administration is done through a web interface, moreover Openfire offers numerous features, there are also several plugins available in Openfire to extend its functionalities.

"BOSTON, Massachusetts, USA—Monday, November 19, 2007—The Free Software Foundation (FSF) today published the GNU Affero General Public License version 3 (GNU AGPLv3).

I’ve talked in the past about a number of “open source” business models. There’s the MySQL “dual license” model, where all the software is available under an open license but a proprietary license can be purchased.

In the past, it seems like there has been a lot of angst surrounding the idea of dual-licensed software gaining any real traction. But after researching this, I think it has a real shot at giving traditional commercial-only software a run for its money. Even if it is simply in the long tail markets

"The Free Software Foundation has published the GNU Affero General Public License version 3 (GNU AGPLv3). This is a new license; it is based on version 3 of the GNU General Public License (GNU GPLv3), but has an additional term to allow users who interact with the licensed software over a network to receive the source for that program. By publishing this license, the FSF aims to foster user and development communities around network-oriented free software..."

The FreeType library is released under a dual license: you can choose either the FTL (FreeType License) or the GPL, version 2. That means that a software stack that uses FreeType has to be license-compatible with one or the other (or both).

"The Free Art license is the English language version of the Licence Art Libre, a French copyleft license for works of art. Created in July 2000, it is the first free license, in the spirit of the GNU General Public License, dedicated to works of art..."