Rockstar's official tool for user-created content in GTA V Online allowed players to create races, deathmatches, and various other modes. We work through it here.
What it did
Race editor. Deathmatch editor. Capture-mode editor. Players could publish content within a constrained system. Different from full modding: yes, it was guided and limited, but it was official.
Popular at launch, but it lost steam over time.