Östermalm Echo Dome

This echo chamber is a large round room with a spherical roof made out of concrete. The spherical roof bounces sound back and forth in a very unintuitive way. It makes it so even the quietest whisper on the other side of the room can be heard perfectly, but people next to you are not more audible at all. Stand in the center of the room, and you'll hear everything that is said around you, and everyone will also hear you. 

Apart from all the practical jokes you can play here, it's amusing to see people look around with a confused face when they walk alone and suddenly hear other people talking, their voices carrying as if they were right next to them.