x11vnc

TLDR

Launch a VNC server that allows multiple clients to connect

$ x11vnc -shared

Launch a VNC server in view-only mode, and which won't terminate once the last client disconnects

$ x11vnc -forever -viewonly

Launch a VNC server on a specific display and screen (both starting at index zero)

$ x11vnc -display :[display].[screen]

Launch a VNC server on the third display's default screen

$ x11vnc -display :[2]

Launch a VNC server on the first display's second screen

$ x11vnc -display :[0].[1]