Here is a design for a full-featured motor shield that will be able to power many simple to medium-complexity projects.

• 2 connections for 5V 'hobby' servos connected to the Arduino's high-resolution dedicated timer - no jitter!
• 4 H-Bridges: L293D chipset provides 0.6A per bridge (1.2A peak) with thermal shutdown protection, internal kickback protection diodes. Can run motors on 4.5VDC to 36VDC.
• Up to 4 bi-directional DC motors with individual 8-bit speed selection
• Up to 2 stepper motors (unipolar or bipolar) with single coil, double coil or interleaved stepping. 
• Pull down resistors keep motors disabled during power-up 
• Big terminal block connectors to easily hook up wires (10-22AWG) and power 
• Arduino reset button brought up top 
• 2-pin terminal block and jumper to connect external power, for seperate logic/motor supplies 
• Tested compatible with Arduino Mega, Diecimila & Duemilanove 

Kit comes with all parts necessary, unassembled. Instructions, examples, and other documentation is available on the Adafruit website. Motors and Arduino are not included.