Godmode (Horde - Level 8+)

The quest The Wolf and The Kodo is right outside Durotar in the hut (pictured below), and after starting the quest you can begin a dialog in which you are transformed into a Timber Wolf. If you start the dialog (Can you tell me your fable, Shin?), and then immediately jump onto a 2-seater, you will become completely invisible. You are able to attack mobs, duel players (but they must start the duel from within your party, as only your party members can see you). Mobs will reset if you attack then, but you can have another person CC mobs or tag them while you attack and will never get aggro.

Above: The questgiver and me on a 2-seater in position for the exploit.

Above: Dueling my friend, who is invisible. Note him in my party and directly in front of me on the minimap.

This can be very good for countering raids on Durotar or attempts at assaulting Orgrimmar.

This only works in Durotar because it get removed after zoning, but it could probably be done with similar quests where you are transformed and can mount on a 2-seater. Hopefully this can be expanded and made more useful.