This is a very simple overdrive effect. There are a lot of same kind of effects on the net, but I decided to design something simple. It's a great beginner project.

This is me playing with it:
http://youtube.com/watch?v=z-1BsJz9Cpk
First I play clean, the switch on the effect.

Step 1: Diode Clipping

Overdrive or distortion occurs, when signal is amplified too much. Amps have an upper limit of volume. If a sound signal is louder than then it's peaks are simply chopped off. That is called clipping (distortion).

We can simulate clipping, by placing diodes between the signal and ground.

Step 2: The Schematic

Now it's time to build one for yourself too!

This is what you need:

6.3mm stereo female jack
6.3mm mono female jack
DPDT switch
0.1uF capacitor
LM386 op-amp
220uF electrolytic capacitor
47k resistor
100k resistor
red LED
two 1N4148 diodes

Incase you don't know, how to read schematics, there are a plenty of tutorials available. Just Google