The project I’m writing up at the moment is a tank level sensor that uses a differential pressure transducer to measure the difference in pressure between the atmosphere and the water at the bottom of the tank, which in turn allows the Arduino to figure out how full the tank is since pressure increases with depth. I built a prototype using part of a circuit board that already had an op-amp comparator / amplifier on it and it worked nicely so I’ve just finished doing a version on a prototyping shield:
That picture show it complete except for two things: the 4-way connector for the lead to the pressure sensor, and a jumper that selects which analogue input the shield will use. Coming along nicely!