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].
OneWire Library
OneWire lets you access 1-wire devices made by Maxim/Dallas, such as temperature sensors and ibutton secure memory. For temperature sensors, the DallasTemperature library can be used in conjunction with this library.