PeterH:
If you use servos instead of motors, you don't need any special driver hardware (they are just controlled from a PWM analog output pin).Depending how many servos you need to control, it may be difficult to drive them all from a single Arduino (difficult but not impossible - but adding multiple Arduinos might be an easier problem to solve than multiplexing servo outputs).
Or you could use one of these http://proto-pic.co.uk/20-channel-rc-servo-driver/
I am using one for client where the Arduino is tied up doing other stuff that messes with the timers and so the PWM. This solution passes PWM signal generation away from the Arduino and is very stable.
Cheers Pete.