MADBOT
This bot was created by Raman and Me. The video explains its working. I should not go into detail.
Controlling Motors using Transistors....
Atmega16 or any other microcontroller cannot supply enough current needed to run even a 5V motor. Try running a 5V motor using any two pins of your microcontroller ( One pin's output being '1' and the other being at '0' ).
So for this purpose we can use an H-Bridge ( L293D IC ), but if you really want to cut down your cost and make your circuit even simpler, you can go for an NPN transistor ( You may of course use a PNP too ).
The circuit is given below for an NPN transistor. This circuit is very simple to understand if you have little knowledge of transistors. When you give a HIGH to the base, then the transistor's emitter-base junction being forward biased, it starts to conduct. So there is a current flowing through the motor and it runs. Main advantage here is that, base current being very small, can be easily provided by the microcontroller.