We usually get a real Christmas tree if we're going to be in town for Christmas. A real tree needs watering though, which is something we've been less than...consistent with over the past years.
I decided to do something about this and build something to alert me when the water level gets too low. Two strips of aluminum foil taped to either side of a piece of plastic provide my water sensor. One strip is connected to an analog input on the arduino, and the other strip is connected to +3.3V. When the sensor is submerged I get a reading of around 300 "units" from the ADC. When it's removed from the water, a 10k pulldown resistor brings the reading down to 0.
I've hooked up a tri-colour LED to indicate various states, and plan to have an audible alert as well.
I'm not sure if the aluminum will end up corroding, nor if I'll be able to power this off batteries for any length of time. Still, I'm pretty pleased with it so far!
Here you can see that LED is green when the sensor is submerged, and changes colours (like a traffic light, as per Thomas' request) when the sensor is removed.