MediaWiki - Hosts documentation of the MediaWiki software, which is used by all the above wikis and hundreds more. It is currently being maintained by the Wikimedia Foundation.
Wikimedia Commons - Hosts free images for use on all other Wikimedia Projects. Feel free to post images that you have taken yourself here, or that are free, and if you have gained permission from the content creator.
Wikidata - The only Wikimedia wiki to use Wikibase. The primary purpouse of this is to provide structured data, and to connect all Wikimedia projects together without the need for Interwiki links.
Wikivoyage - Hosts free travel guide information.
Meta-Wiki - Used for co-ordination within the Wikimedia network. This is the place to propose new languages of projects and to propose new projects.
Wikimedia Incubator - A place to develop projects in new languages (where the corresponding wiki does not exist), before they are proposed at Meta.
Wikimedia Labs - Allows users to host tools that are used to edit Wikimedia project, run automated programs, or present information based on the content of a Wikimedia project.
Wikimedia Foundation - A restricted wiki that hosts Wikimedia's official policies and terms of service.