This circuit controls a small 12 Volt pump that is used to fill a toilet tank from an external rainwater collection cistern.
http://www.solorb.com/elect/solarcirc/tankfill/index.html
can be used to also measure almost any liquid.
http://www.imagineeringezine.com/PDF-FILES/capgage.pdf
uses an ac-sensing signal to eliminate electrolytic corrosion on the probe.
http://www.uoguelph.ca/~antoon/circ/sensor3.htm
http://www.electronicsforu.com/electronicsforu/Lab/freecircuitslist.asp?id=247
http://www.zen22142.zen.co.uk/Circuits/Alarm/water_lev_alm.htm
prevents overflow of water tank
http://www.citycom.gr/electronics/downcircuits/63.zip
uses bead thermistors sticked to to the tank as sensors
http://www.mitedu.freeserve.co.uk/Circuits/Misc/hwtank.htm
method of measuring the height of fluid in a column by using ultrasonic waves
http://www.e-insite.net/ednmag/index.asp?layout=article&stt=000&articleid=CA92008&pubdate=7/19/01