Since I’ve bought a uni-polar motor then I need two ‘switches’ per coil – as per my diagram earlier. However: I will forget about the center tap for now and just drive it as if it was a bi-polar motor. So I’ve chosen to use a L293D chip see http://focus.ti.com/docs/prod/folders/print/l293d.html
This contains four half H-Bridge ‘switches’ – so one chip can drive both coils on the same motor. One chip per motor.
I’m not going to use the ‘enable’ pins on the L293D so I’ve just connected them to pin 16 (the +5v logic supply). Each of the four input pins just maps to the equivalent (higher current) output pin.
Check the datasheet to make sure that the L293D can provide enough current for the motor you are using. NB if you need more current then the L293 supplies twice as much but you MUST then connect external diodes as shown in the datasheets. If that still isn’t enough then search this site, or Google, for other higher current H-Bridge circuits.
Here's how I connected the L293D