Supplier Link: (unknown)
A simple three PIN device that is good for binary (on/off) short-range detection between 0-30mm. Uses a POT to adjust the threshold. This came with the Arduino 4WD Robot platform.
Freetronics LCD And Keypad Shield
Part Number: SH-16X2LCD
Supplier: Freetronics, Jaycar
Price: US$29.95 (as of 2013)
The Freetronics LCD And Keypad shield is a very popular display/UI shield for the Arduino.
The Arduino comes with a LCDCrystal library, making it almost too easy to use the LCD display (see the minimal code example below).
// Code example taken from the Freetronics website
LiquidCrystal lcd( 8, 9, 4, 5, 6, 7 );
// your main loop code here...
Ultrasonic Ranging Module HCSR-04
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
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.