Steam Workshop

Steam Workshop is a feature of the Steam client developed by the Valve Corporation. Steam Workshop allows users to create and share custom maps, weapons and other modifications to a supported video game. Steam Workshop was originally enabled for Team Fortress 2, and later expanded to other games such as The Elder Scrolls V: Skyrim, Portal 2 and Dota 2.

Counter-Strike: Global Offensive
The Counter-Strike: Global Offensive workshop, on top of submitting custom maps, also allows users to submit weapon skins. These skins can be voted on, which, depending on popularity, may be included in the game in a later update.

Paid mods
In April 2015, Valve starting allowing developers to generate revenue from their workshop items. After bad reception and misuse, the feature was quickly pulled.

Games using Steam Workshop

 * This is an incomplete list, please see this link for a complete list.