For me it was a little known game called ThinkTanks. It was created as a demo for a companies game engine and never got much support. It was a silly little catoon tank game with basic 3D.
The community figured out how to bypass the demo restrictions on the game engine. The game was modded to an insane degree. Thousands of new maps, new game types and objects. I dabbled with it at the time and created around 70 maps personally.
Last I check the original game was someone like 8mb of space. The full mod pack with all of the maps was 2GB