There is never a shortage of projects that involve modifying inexpensive remote-controlled cards to work with an Arduino - and doing so is easy with a DC motor control shield. However we feel that this project rates a mention due to the quality of the control system. The Arduino in the car has a typical Bluetooth-serial adaptor, which allows for receiving serial data from an Android or other device.
However the quality of the Android app that controls the car is very good, and allows for not control direction and speed control - but also LEDs for headlamps and brake lights plus other options. You can see a quick demonsatration of the system in the following video:
If you're looking into controlling a pair of DC motors(or a stepper motor) from your Arduino or compatible, check out our new HBRIDGE: DC/stepper motor shield. Based around the powerful Allegro A4954 H-bridge driver IC you can control two DC motors with complete ease, or one bipolar stepper motor. With connections for external power management, a complete beginners' guide and documentation - motor control couldn't be any easier. For more information and to order, visit the HBRIDGE: page.