Like to keep track of when it has last rained? With this tutorial from theorycircuit you can learn how to interface between an Arduino and a rain sensor.
Once you have done this the possibilities are endless. You could record data about how often and at what times you get rain, or even use the rain sensor as an input to your home automation system. For example you could automatically fire up your heater or close your windows when rain is detected! To get started learning about detecting rain with Arduino check out the following link.