See the Amazing AirJelly! Watch it Swim Through Air

"Mesmerizing floating robo-critter: AirJelly"
MAKE: Blog (April 29, 2008)

"This "AirJelly" floating mechanical creature is a joy to watch:

"AirJelly houses two lithium-ion polymer accumulator batteries rated at 8 V and 400 mA, which can be completely charged in half an hour and are AirJelly's sole source of power. ..."

I'm not sure that I'd call this gadget a "robot," but it certainly is a remarkable mechanical mimic of the basic jellyfish design.

The video (1:40) is impressive, and has some 'mesmerizing' music that fits with the AirJelly's languid behavior.