License

Almost all content of almost all Wikimedia projects is dual-licensed under CC BY-SA 3.0 and GFDL (according to Terms of Use § Licensing of Content). The projects migrated from GFDL to CC BY-SA 3.0 in 2009 (Licensing update), because GFDL has some annoying requirements, but despite that, new contributions are still required to be made available under GFDL.

There are several projects that are exceptions:

Wikinews projects are licensed under CC BY 2.5 only (according to configuration of wgRightsUrl).
Hungarian Wikinews is licensed under CC BY 3.0 only (according to the same).
Wikidata is licensed under CC BY-SA 3.0 only (according to configuration of wgRightsPage).

There are also several kinds of content that are exceptions:

Content of Wikidata main namespace, Property, Lexeme and EntitySchema namespaces is available under CC0 (https://www.wikidata.org/wiki/Wikidata:Copyright)
Content of Wikimedia Commons structured data about files is available under CC0 (I'm struggling to find a page that clearly documents this…)
Content of MediaWiki.org help namespace is available under CC0 (https://www.mediawiki.org/wiki/Project:PD_help)
Files uploaded to all wikis are available under a variety of licenses, as specified in each file's description
Tabular data and map data on Wikimedia Commons in the Data namespace is also available under a variety of licenses. Each piece of content documents this with the license attribute (by default CC0).
(there may be others)