Using the Arduino development platform you can modify a lot of existing products, toys and other devices to work with or be controlled by an Arduino or other device. Doing so isn't that difficult, and an interesting example of doing so has been demonstrated by Instructables user "malnas01" who shows how a typical remote-control car can be controlled via a personal computer via Arduino.
The first step is create a motor-control circuit which allows one Arduino to control the speed and direction of the motors and any steering servo if necessary - and also has an inexpensive RF wireless receiver. Another Arduino connected to the PC accepts serial text from a simple Java applet or other program and sends these as wireless commands to the car's Arduino which interprets the text into car controls. This is demonstrated briefly in the following video:
As you can see, commands can simply comprise of single characters which are easily transmitted using wireless RF modules - and to learn how it's done, visit the project Instructable. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're looking to work with Arduino and RF wireless hardware that integrates neatly with the VirtualWire library, but don't want to make your own receiver circuit - check out our range of 315/433 MHz receiver shields:
Apart from being idea for working with the various low-cost data links on the market, the shield can also be used to capture wireless weather station data, as described in the book "Practical Arduino". For more information and ideas, check out the product page.