The Arduino Motorshield is the missing link for making a robot with the arduino. This Arduino Motorshield allows you to connect 2 DC motors and have total control of them from the arduino.

It allows you to move each DC motor
- Clockwise
- AntiClockwise
- Stall
- Control motor speed

This motor shield allows Arduino to drive two channel DC motors. It uses a L293P chip which
deliveries output current up to 2A each channel. The speed control is achieved through conventional PWM which can be obtained from Arduinos PWM output Pin 5 and 6.

Specs:
Peak Output Current (each Channel)
- Non Repetitive (t = 100ms): 3A
-Repetitive (80% on -20% off; ton = 10ms): 2.5A
-DC Operation: 2A
Analog voltage input: VIN 6.5-12V PWRIN 4.8-24V 
Digital voltage input: VD = 5V (7V max)
Max power dissipation: 25W

See  Motor Shield Tutorial for details of how to use it.