This a simple circuit demonstrate the Tiny family high speed PWM capabilities to produce a sinewave output. It was also incorporated functions to play Piano Key Tones using sinewave waveform, as a smooth and clean sound as the ones produced by cords.The sinewaves produces are better in low frequencies. source

A two LED flasher is about as simple as it gets.A simple LED flasher is a good project to get started with AVRs. This ATtiny13 based circuit alternately flashes 2 LEDs (or however you want them flashed). Note: Circuit and firmware now available.source

The circuit is rather simple. Main parts are the microcontroller, the light sensor and the RGB-LED. The sensor and R4 are forming a voltage divider. An ADC (Analog Digital Conversion) channel at pin 3 is used to read the values of the sensor. The circuit is designed for 5V power supply. It has no integrated power regulator.source

This circuit slows the speed of a conventional proportional servo.It regulates the time required for full servo rotation from 1 to 10 seconds without any reduction in the servo’s torque, and every servo position is possible (proportional). source

This is a simple IR receiver circuit which plugs into a serial port of a computer. Okay, there are many other circuits of this kind, and most of them are even simpler, but this circuit has two major advantages: (1) it uses an Atmel AVR RISC microcontroller (an AT90S2313) instead of the usual PIC microcontroller and (2) it uses a Maxim MAX232 for the generation of valid RS232 levels.source