Go-Mod is a sandbox modification for Half-Life based on the popular Source Engine game Garry's Mod.
gm_somewhere by tnk_x5000. It's an open map. With a lot of space to mess around.
ca.wad (the "Citizen Arms" (by tnk_x5000) texture pack)
There is no furniture. You can add it by yourself.
I can make the map way better than this, but this way it's not so laggy, although it's a big open map.