pH is really important environment parameter. It’s value expresses acidity or basicity of a solution, e.g., soil or water. pH parameter depends on concentration of H+ ion in the aqueous solution. Probably we al heard that human skin pH is 5.5 so most cosmetics tend to be close to it to keep natural balance. So where would you use a pH meter? Probably many places like fish tanks where special fish can not stand different pH level, probably flower soils, any water and other purposes you can think of.
phduino is a great choice for measuring pH level as it is an open source project. It has a LCD for displaying pH value and temperature. Phduino uses a standard glass electrode pH sensor. Phduino is the same Arduino board that has all matching Arduino pins and additionally LCD connector, temperature sensor connector and several other components specially needed for operation. With all this you get pretty cool tool that can be expanded for many purposes by interfacing, programming new features.