Spectator

Spectator mode is a game mode which allows you to fly around to watch the world without interacting with it.

Features
While in spectator mode you can fly, clip through blocks and entities, and cannot interact with blocks.

Spectators are always flying. They cannot stop flying by landing, as the player will just go through the blocks. The mouse scroll wheel, the sprint key, and the slowness and swiftness effects all affect the flying speed.

You can move through blocks, entities, and other players. Spectators are not noticed by mobs, and do not trigger spawners or spawning, though a spectator's presence will keep a mob from despawning.

While in spectator mode you cannot take damage, as all blows go through you. Lava, drowning, fire, and suffocation also do not affect you. Though, you can take damage from the void or the command.

If you open your inventory, you cannot interact with any of the items in it. Likewise, viewing GUIs is possible, but it is not possible to interact with them. You also cannot pick any item up. Items in your inventory cannot be used.

If you left click on a mob, you take their view. Your position is locked to that of the mobs, and your POV moves in accordance with the mob. Viewing oneself while spectating from an entity's point-of-view using shows the entity. Pressing the dismount key ( by default) returns the player to flying mode. Certain mobs also apply a shader while looking from their point-of-view. This filters how the world is portrayed to the spectator. For instance:

Spectators can see other spectators and invisible mobs, as well as themselves using. Spectators appear as a translucent disembodied head, and the selected item from the hotbar is also visible. Invisible mobs appear translucent, like when viewing a invisible ally with set to true.

Trivia

 * Using on a spectator will cause the item(s) to drop at their location.
 * Player selectors (in commands) will target spectators
 * If you put a pumpkin on your head before going into spectator mode then when you enter it your vision is still filtered even when you enter the view of a mob.