Just follow the picture exactly in less than 1 minute and you have to download a source code and make this thing work.
http://zedomax.com/blog/zedomax-diy-109-build-a-digital-thermometer-in-one-minute/
This one is using a 16F876 PIC, MCP1047A temperature sensor ( X2 ), MCP1541 voltage reference and MCP6022A opamp.
http://www.geocities.com/constantinescuradu/content/thermo.htm
http://www.uoguelph.ca/~antoon/circ/auto-fan.html
Sometimes we have a need to measure the temperature of a room, of an object or just maybe a terrarium;
http://www.e-dsp.com/how-to-measure-temperature-with-the-dallas-maxim-ds1820-sensor/
Using a thermistor in the position shown makes a heat activated sensor. A change in temperature will alter the output of the opamp and energize the relay and light the LED. Swapping the position of the thermistor and 47k resistor makes a cold or frost alarm.
http://www.mitedu.freeserve.co.uk/Circuits/Misc/tempalarm.htm
It is based on an AT90S2313 and a Dallas DS1621 Digital Thermometer. The temperature is displayed on a dual 7-segment display, and two buttons are used for setting parameters. A high current relay switches the heating element.
http://www.myplace.nu/avr/thermo/index.htm