Steam Workshop

Before Steam Workshop opened in 2011, modding almost always meant hunting down files in a forum thread and copying them into the right folder by hand. Workshop turned that into a button: Valve let players publish content and others subscribe, after which Steam kept everything updated automatically.

It launched with Team Fortress 2, and the model spread quickly to Dota 2, Counter-Strike and thousands of other titles. For some games, cosmetic items from the Workshop became an economy of their own, with creators taking a cut of sales — part of why modding went from hobby to profession for a lucky few.