I ordered some time ago Arduino Sensor Shield v4.0 because it is an easy way to connect input and output devices to Arduino. Arduino Sensor Shield provides three pin (ground, voltage, signal) interface for connecting all kinds of sensors (potentiometer, button, LDR etc.) and output devices (relay cards, servos etc..) to Arduino.
The Arduino Sensor Shield v4.0 product came without any documentation. Fortunately SensorShield page at Wikispaces gives a good description what you can do with the sensor shield (I checked that before I bought Arduino Sensor Shield v4.0). I will not start to rewrite that material to it. Here is a good picture from SensorShield page that shows clearly the Arduino Sensor Shield v4.0 connections:
This Arduino Sensor Shield v4.0 did what is promised to do well at least for three pin input and output sensors (I have not tested other function). This was well worth of the price US$6.30. Just plug the board on top of Arduino board and start experimenting. It makes much easier to connect many sensors than just wiring them to Arduino board connector (very quickly becomes a hard to manage wire mess).