It's Here! Chris Odom's Physical Computing & Robotics with the Arduino IDE - Volume One

Non optical single relay board

$1.25

These guys are okay... They work fantastic on older 5 volt systems like the classic Arduino board but....they need some care on the newer lower voltage boards. If you are going to use them on a 3.3v signal pin, they must have a 3.3v supply as well in Vcc!