For this project the author used the LM35 from National Semiconductor which is a precision temperature sensor. It has a range of -55ºC to +150ºC and a accuracy of ±0.5ºC. In this project the output voltage of the LM35 is converted by the AD convertor of the AT Mega8 and then displayed on an LCD. Also in this example the range has been set from 0 to 45 ºC with a resolution of 0.5. If you would like the reading to be carried in Fahrenheit you can use the LM34.
AVR and LM35 Thermometer: [Link]