AdAge writes about tools that let players create their own games: "What it is: The next wave of user-generated content?" No. Levels, quests, environments and entire games are some of the oldest forms of digital "user-generated" content. The other such form is pr0n.
Above, a map editor for Doom that came along with the game. Below, a "user-generated" map for the Counter-Strike Source shooter featuring a McDonald's restaurant: