Arduino is a great starting point for electronics, and with a motor shield it can also be a nice tidy platform for robotics and mechatronics. Here is a design for a full-featured motor shield that will be able to power many simple to medium-complexity projects.
1.2 connections for 5V ‘hobby’ servos connected to the Arduino’s high-resolution dedicated timer – no jitter!
2.Up to 4 bi-directional DC motors with individual 8-bit speed selection (so, about 0.5% resolution)
3.Up to 2 stepper motors (unipolar or bipolar) with single coil, double coil, interleaved or micro-stepping.
4.4 H-Bridges: L293D chipset provides 0.6A per bridge (1.2A peak) with thermal shutdown protection, 4.5V to 25V
5.Pull down resistors keep motors disabled during power-up
6.Big terminal block connectors to easily hook up wires (10-22AWG) and power
7.Arduino reset button brought up top
8.2-pin terminal block to connect external power, for seperate logic/motor supplies
9.Tested compatible with Mega, Diecimila, & Duemilanove
More info: Shield Details