A new snapshot has been released for Minecraft and with it comes a fairly massive thing, just for the map-making part of the community: an official 'spectator mode'.
Spectator modes are useful for those game modes/maps where you don't want players who are 'out' to interfere with the game for the rest of the players, but equally don't want to box them all up in a 'waiting room' with nothing to do. The new official spectator mode can be turned on by typing the command '/gamemode 3' or '/gamemode spectator' and makes it so that spectating players are invisible to all but other spectators, can check inventories but not mess with them, inhabit the bodies of mobs (although not control them) and pass through blocks. In other words, it is a gamemode that turns players into ghosts.
Other notable changes include activator rails now being able to eject entities from minecarts, a new 'invisible block' for walling off areas of maps and the book copying can now differentiate between the original copy and copies of a book.
Snapshot notes are here.