Any project about any topic under licenses which permit the use, copy, modification and distribution of the knowledge or information generated in this project. That is, these kind of projects release free/libre knowledge. Then you give these freedoms:
The freedom to use the knowledge generated in the project, for any purpose.
The freedom to study how the project works, and adapt it to your needs. Access to the project documentation and any other information produced is a precondition for this.
The freedom to redistribute copies of this information so you can help your neighbour.
The freedom to improve the project, and release your improvements to the public, so that the whole community benefits.
Access to the project documentation and any other information produced is a precondition for this.
Under this definition, for us any free software project is a libre project. A project about Architecture or Ecology or another topic with this philosophy, is also a libre project.
Sharing objectives, work and results
Libre projects join people with similar objectives (or problems), and they share the work, and release the results publicly and with freedom.
For make this cooperation we rely on Internet and on existing collaborative working tools oriented to projects (for instance, gforge.org) and legally we rely on Copyleft.
That is, Internet, Free Software collaborative tools and Copyleft made a basis in which we can develop any kind of Libre Projects about any kind of topic, in a way not possible some years ago.
More info in the article How to create a Free Project.
Examples of Copyleft licenses that can be used for Libre Projects:
For Free Software:
GNU General Public License
For Free Documentation:
GNU Free Documentation License
Open Publication License, Version 1.0
The Design Science License
For Free Art:
Free Art license
Creative Commons, Attribution-ShareAlike License
For Libre/Open Designs:
Libre Designs General Public License