gm_flatgrass

gm_flatgrass is, literally, a square acre of grass texture broken only by large elevated concrete platform in the middle of the map. There are no artificial boundaries that separate the playable area from the skybox (or lack thereof) and has no other distinguishing landmarks other than the aforementioned platform. 
 
The map comes default with Garry's Mod and is most popular on severs dedicated to building and free-roaming since it has no distractions or elements that could crash the game/server, aside from the actions of actual players. Like other built-in map, gm_construct, enhanced versions of the map have been created by the community. Some of these maps have a larger area for more complex constructions. Some enhance the map like adding recognizable boundaries, changing the middle section, or adding additional functions. Some have even made maps based on the overall concept of a large, empty map; including replacing the grass with a large body of water.