Civilization V joins Skyrim and Portal 2 on the Steam Workshop section of Steam, a clearinghouse for user-created content. At launch, players had access to map packs such as Middle Earth and Westeros (the main continent in Song of Ice and Fire), additional, wonders, and religions such as Jainism, Baha’i, and Pastafarianism. All downloads are free (though you do need to own the actual game itself), and users are able to rate and comment on individual mods, as well as create custom packs of mods to share with others.
You can check 2k Games’ announcement or go directly to Civilization V’s section of the Workshop to check out what’s available, either in a web browser or your Steam client.