Here is a finely made digital clock with calendar and thermometer.
It is based on Microchip PIC16F628 (or PIC16F84) microcontroller and DS18S20 or DS18B20 temperature sensor from Maxim-IC. The hardware is pretty easy because it contains only a PIC microcontroller, DS18S20/DS18B20 sensor, 4x7 segment LED display with common anode, four 2N3906 transistors and several resistors.