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This inexpensive motor controller board http://www.raspberrypi.org/archives/5269 gives full control over 2 motors and is one of the easiest ways to get into Robotics on the Raspberry Pi.
To use it in ScratchGPIO, simple set variable AddOn to RTKMotorCon and then you can use 2 variables – Motor1 and Motor 2 and vary the speed and direction of the motors from -100 to 100.
It also breaks out the 3.3V, GND and the I2C pins so it is very easy to add on other I2C devices as well.
(Support currently only available in the ScratchGPIO dev version https://cymplecy.wordpress.com/2013/11/26/scratchgpio-development-testing/ )