The DS18B20 is a very small thermometer which can be easily hooked into the Arduino MCU through any digital input! It requires very little in the way of additional support, a couple of resistors and some hookup cables and you're set to go.

For more information on the DS18B20 check out [Maxim-IC].

Your Arduino can tell you.  With an Arduino board and a DS18B20 digital temperature sensor, you can easily build a digital thermometer, a thermostat to turn devices on/off at certain temperatures, a safety device to power off when something gets too hot, etc.