Very easy to interface to, due there being a native Arduino API function called pulseIn(), which returns the length of a digital pulse measured on one of it’s pins. Example firmware code for this module can be found on the xAppSoftware Blog. Included in the Arduino 4WD Robot Platform.
The DealExtreme Robotic Platform
Product Link: (http://dx.com/p/multi-function-4wd-arduino-robot-raider-car-kits-128715, as of Dec 2017, URL is unavailable)
Price: US$71.70 (as of 2012)
A cheap, versatile, easy-to-use (well, kind of, but read the next paragraph) robotics platform.
No instructions come with this! You are left on your own to figure out how to assemble and then use it. That said, you can find instructions written by users for most of the individual components on online blogs.