Freecam mod it is a Forge modification that allows you to FLY as a SPECTATOR though the minecraft world. It works even with ONLINE servers. Freecam helps to explore world underground or built factories. You may fly through any blocks in any way.
Online servers, other players and mobs will see you as standing in one place.
To enable/disable FreeCam mode press F6
Better way to use it with the brightness mod ClearVision (F7 to activate ClearVision)