Officially moved from experimental mode to the main game.
To enter the mode use the command /gamemode spectator or the slider in the settings
Interface and indicators are getting smaller
Player items do not disappear or change when activating or deactivating Spectator Mode
Spectator Features:
Fly and pass through all objects in Minecraft
Cannot take damage or be affected by anything
Cannot use or interact with items
Totally invisible
Can stay awake for the night
Only the head remains instead of the full model
Generate chunks if no one has been there before
Can't spawn mobs
Respond to certain commands