Adafruit ULN2803: 8 Channel Darlington Driver (Solenoid/Unipolar Stepper) [ADA970]
Bring in some muscle to your output pins with 8 mighty Darlingtons! This DIP chip contains 8 drivers that can sink 500mA from a 50V supply and has kickback diodes included inside for driving coils. This will let your little microcontroller or microcomputer power solenoids, DC motors (in one direction) and unipolar stepper motors.
Please note, this is an 'open collector' driver - it can only be used to connect the load to ground and there will be a 1 Volt (or more) 'drop' across the internal transistors. The inputs can be driven by 3.3V or 5V logic. Fits nicely in any breadboard or perfboard.