IO Expansion shield is one of the market favourites. Fast robotics prototyping made affordable and simple thanks to this design. Soldering and wiring often becomes a problem when a bridged connection happens and components break. Increasing time a costs for projects. Often, without this solution and a soldering station, wire cripping tools or soldering station, robot prototyping or electronics DIY becomes a very long term project.
The DFRobot Expansion shield introduced to the market the famous color code for sensors and actuators input and output. The 3 pin format for Signal, Voltage and Ground is extremely useful, specially if used altogether with our increasingly large range of modules, sensors and devices that just fit .
Alternatively, the board includes power input for salvaged power supplies or laboratory power supplies. Very common Xbee socket for multipurpose wireless connectivity such as RF, wireless, bluetooth.
The new version includes a voltage hardware setting through a jumper to allow compatibility with a bigger range of components at 3.3v like Raspberry pi, Due and others.
It also includes a Servo external power, with a protective diode. This way you can enable a large range of servos from your Arduino.
Latest version features a convenient switch that lets you program your ATmega328 boards like UNO, while you have another Serial device connected like a Xbee. Switch it, program it and get it back up and running with another switch. You don't need to disassemble your hardware or wiring.
A LED on top board and a reset button is in order to make it just a natural extension to Arduino boards. The LED is connected to pin 13.