What types of fuel quantity sensors are used in an aircraft? How do they work?
From this PPRune forum thread, the Boeing 777 does use ultrasonic sensors (some 20 per tank)


  This airplane has some 20 sensors per tank. Each of these is effectively a "radio doppler" and works on the same principle. The sensor calculates the height of fluid by measuring the difference in the speed of the sonic signal sent out thru one sensor to another,as against the ACTUAL local speed of sound (which is a preset value) thru standard temp air.This difference is then converted very accurately into height in the tank in mm.Which translates into quantity of fuel AT THAT sensor given the actual specific gravity of fuel at that point.
  - King on a Wing