A vibrator motor is a small motor with off-center weights that make it wobble and vibrate. They are used in cell phones, pagers, gaming controllers and personal massagers for haptic feedback.
This type of vibration motor is easy to make and can be used on all sorts of electronics projects, from simple bots to a fully-fledged robot. You can find these at many of your usual electronics sources, including Digi-key and Precision Microdrives.

To make a simple vibrating motor, you simply need a standard DC motor and an off-center weight. You can attach this to the motor's shaft with a terminal strip or clamp something like a pencil eraser onto the shaft. You'll also need a way to control the speed at which it spins.
The amount that it wobbles can be changed by the distance from the shaft to the weight and the speed at which it spins. This can be altered by using a transfer function, and changing the force that is applied to the motor.
These motors are a great way to add a little extra pizazz to your project, while still staying on the budget. They are easy to 3D print, and can be secured directly on PCBs or into enclosures for a more permanent installation. They are especially good for quick prototypes that are not going to be mass produced or need to stand up to a lot of use.