There will be never enough of thermometers as we know. There are tons of projects with different sensors and displays. You can choose the one which suits your needs best. SO here is another simple to built thermometer by using LM35 temperature sensor. Temperature sensor gives 10mV per ºC. This means that at 25ºC the output will be 250mV. If you connect this sensor directly to 5V referenced MCU with 10-bit ADC accuracy, you get 4.9mV step or 0.49ºC resolution. In order to increase resolution there is an operation amplifier used which adds gain of 11 to sensor signal. Gain increases resolution by order of magnitude – to 0.0445ºC.
Attiny26 reads temperature value and displays it on 4-digit 7-segment LED display. LED display is common anode where each anode is driven with transistor key. Microcontroller simply scans through digits at 60fps rate to make impression that all digits are light at once. Power supply is provided by using 7805 voltage regulator which drops voltage from 9V to 5v. For longer battery life there could be switched DC/DC converter used.