In addition to Wikipedia, the Wikimedia Foundation runs several other projects to produce free-content reference material. Commonly known within Wikimedia as sister projects, sibling projects, or simply the projects, the nine current Wikimedia projects are Wikipedia, Wiktionary, Wikinews, Wikibooks, Wikisource, Wikiquote, Wikispecies, Wikiversity, and the Wikimedia Commons. Each project handles a different type of material, from textbooks to images, and each has a separate group of volunteers working on a dedicated wiki, usually with many language versions. The Wikimedia Foundation also coordinates the MediaWiki software that all of the projects run on. Often thought of as the tenth sister project, the MediaWiki software development process will be described more in the next chapter. The Meta-Wiki site, also described in the next chapter, serves as a coordination site for all of the projects and languages. Volunteers produce the content for all Wikimedia projects using a wiki model similar to Wikipedia's, and all of the projects license their content under the GFDL or another free license.

These different wiki projects all complement each other. The intention is to separate out types of content, such as dictionary definitions, encyclopedia articles, and archival material, while allowing crosslinking between the projects topic by topic. For instance, you would add quotations related to Abraham Lincoln to Wikiquote, and the Wikipedia article [[Abraham Lincoln]] can link to that store of quotations, rather than trying to include them all. With over 500 sites involved—nine sister projects, each with a large number of language versions—a systematic naming system to enable linking is obviously needed. We'll explain how this works in "Linking Between Projects and Copying Content," on Section 3, “Linking Between Projects and Copying Content”.

All of these projects use the MediaWiki wiki software, so you don't have to learn new editing processes to contribute to them; all you need to learn are some different namespace and page structure conventions. Don't expect every project to have the full development seen now on Wikipedia, however, because the projects were started at different times and develop at their own (sometimes leisurely) pace. Additionally, you shouldn't assume that Wikipedia culture (particularly English-language Wikipedia culture) transfers automatically to these other projects. Each project, and each language version of each project, can and should be thought of as a community in its own right. Each has its own policies and variations in how processes, such as deletion debates and promotion of administrators, are handled. Watch for and learn the norms of each new project when you start to participate. But each project has some familiar features: help pages and some variation on a community portal and a village pump for discussing and coordinating the project. Just as on Wikipedia, these are good places to begin learning more. Wikimedia projects all share some key values, too. Civility and assuming good faith are always important when interacting with other editors, and copyright violations are never acceptable on any of the projects. And naturally, all projects are open for editing by anyone. Just find a task you want to work on and start editing, either anonymously or logged in with a user account.

In this chapter, we'll describe each of the sister projects in turn. The default license is the GFDL; however, we'll note when another license may apply. Following that, we'll describe how to link all of these projects together (and link to them from the English-language Wikipedia) and how to move content between them. Finally, we'll briefly discuss non-Wikimedia wikis.