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 with high-resolution dedicated timer - no jitter!
• Up to 4 bi-directional DC motors with individual 8-bit speed selection (so, about 0.5% resolution) 
• 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 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 (Diecimila/Duemilanove only)
• 2-pin terminal block and jumper to connect external power, for seperate logic/motor supplies
• Tested compatible with Diecimila/Duemilanove (NG is next!)
• Download the easy-to-use Arduino software library, check out the examples and you're ready to go!

Kit comes with all parts necessary, unassembled. Instructions, examples, and other documentation is available on the website

Motors and Arduino are not included.