Using a CMOS version of the 555 timer, this circuit can be used to control the speed of a motor by adjusting the duty cycle of the pulses sent to the motor. source

This is DC motor speed control based on PIC16F867.The circuits use frequency PWM about 10KHz which does not make a noise in the motor.This project can drive 1 Amp motor current without over heat.The motor speed controled by potentiometer which is connected to ADC input pin.source

The project helps you control two fan’s to cool whatever you need by adjust the trimpot. It works by cooling with the first fan when the sensor detects a user adjustable temperature, and it turns the second fan on, when the temperature exceeds 5 degrees over the user adjusted temperature.Each fan turns off when the temperature drops 5 degrees below its turn-on temperature. For example, if the trimpot is set to 80 degrees, the fan turns on at 80 degrees and turns off at 75 degrees.The circuit controlled by ATtiny45.source

This circuit designed by Cook.It can switch a fairly high amount of current, an IRFZ34N MOSFET can handle over 35 Amps if connected to a proper heat sink. Higher power FETs, such as the IRFZ48N or IRF1010Z can be substituted if even larger currents are required. It is also possible to connect multiple FETs in parallel for even more current capacity. Always use thermally conductive grease between the FET and the heat sink, and remember that the heat sink is electrically live. source