A vibrating motor is essentially a motor that is improperly balanced. In other words, there is an off-centered weight attached to the motor's rotational shaft that causes the motor to wobble. The amount of wobble can be changed by the amount of weight that you attach, the weight's distance from the shaft, and the speed at which the motor spins.
This type of motor can be used affixed to all kinds of objects, which will cause them to vibrate and move freely about. This is a quick and dirty way to get a Simple Bot to move about, but not exactly the most elegant.
Vibrating motors can be found inside cell phones, pagers, gaming controllers, and personal massagers.
In absence of those, you can easily build your own vibrating motor by attaching any off centered weight to any motor shaft. They can also be created by breaking in half balanced components already attached to motor shafts.
Follows are some simple examples.
Step 1: Find One
A great source for vibrating motors is inside gaming controllers with "rumble" feedback.
Simply take apart the gaming controller and free the motors. They should be ready to use.
Step 2: Unbalanced Fan
A quick and dirty way to make a vibrating motor without any extra parts is to take a computer fan and snap off half of the fan blades with a pair of pliers. This will make the fan off-balanced and vibrate.
Step 3: Add Eraser
Another simple way to make a vibrating motor is to stick a pencil eraser (or cork) onto the shaft of any standard reversible DC motor.
Step 4: Terminal Strips
If you would like to get fancier about making a vibrating motor, you can clamp a terminal strip to the shaft of a DC motor. The terminal strip itself will be enough to make the motor vibrate. However, to experiment with adding more off-centered weight to your setup, you can clamp small items like bolts into the terminals on the strip.
Step 5: Add Alligator Clip
If you want to get fancy about doing this, you can add an alligator clip.
Find a motor with a plastic nylon gear. Clamp an alligator clip onto this gear. Finally, solder the alligator clip's halves together. This will both melt the alligator clip into the gear and fuse the two halves to prevent it from opening again.
This is now a vibrating motor with a firmly attached off-center weight.