Communication between your android device and arduino via Bluetooth + Tutorial
Bluino is an andoid application that communicates via bluetooth to arduino to control a toy car, more than just controlling the direction (forward, reverse and turning) it specifies the speed to arduino to parse it and using PWM functionality it will modulate the speed.
Project site: http://itmexicali.webs.com/
In this site you will find the arduino source code and how to customize any RC car to make Bluino work.
Download