In a server running the Roleplay modification, you have money and tools which enable you to sell items on the map, or to interact with other players.
There are a large number of Lua scripted 3rd party modifications for Garry's mod. It's an incredibly successful mod for the game Half-Life 2 that essentially converts it into a giant sandbox powered by Lua.
Garry's Mod is a fascinating study in guerilla programming.