Help: Making a device to measure the amount of liquid an a container
What kind of a container are we talking about here?  What kind of liquid?  Is it enclosed?  Under pressure?  Corrosive?  

My vote would be for some sort of proximity sensor at the top of the container that measures the distance to the surface of the liquid.  This means the sensor should not come in contact with the liquid.  Depending on the liquid, it might be possible to use an optical sensor such as a Sharp analog distance sensor, in which case the sensor can peer into the container through a window.  This may not work if the liquid is clear or absorbs infrared light.  Alternatively, you can use an ultrasonic range finder to measure the distance to the surface.  

A capacitive sensor of some sort might also be a good option.  String an insulated wire from the top to the bottom of the tank and measure the capacitance.  Different liquid levels will change the capacitance on the wire.  You can also try this with a pair of wires and measure the capacitance between the two.  There are capacitance to digital chips that you can buy or you can use the capacitance to form a free running oscillator and then measure the frequency.