This just in: Steam Workshop is expanding and Civilization V is its latest addition.
You're probably wondering what in the world that is, and I'm about to blow your mind. Steam Workshop is a platform for creating, sharing, and downloading modded content for a variety of games. Think of it as a a streamlined mod hub where downloading and installing mods for your favorite games is easier than ever before.
If you're into creating content, an SDK is readily available through Valve's website which provides the powerful tools needed to realize your imagination. In Civilization V's case there are map packs, scenarios, and assets that add some free new fan-made content to the game. There are already dozens of mods available including a Middle Earth Map Pack for Lord of the Rings fans, and an ingame editor for those looking to toy—or cheat—around.
Currently the only games supported by Steam Workshop are DOTA 2, Civilization V, Dungeons of Dredmor, Naval War, Portal 2, The Elder Scrolls V: Skyrim, and Team Fortress 2. However, many more will be added soon.