When I ordered my stepper motors sometime last year from motion control products I chose the cheapest model at £11.23 which was FL42STH38-1206A sometime afterwards I realised that these steppers have much lower holding torque than the recommended mendel FL42STH47-1684A-01 (25.5N-cm vs 43N-cm) however this was not a major problem since mendel requires that “Torque must be at least 0.13 Nm” which is 13N-cm, and well below the rated holding torque. It was only when I came to wiring the steppers up that I came across the next problem. The Generation 3 stepper drivers are for use with Bipolar steppers.
Wheras the steppers I had bought were Unipolar steppers.
Fortunately for me they were 6-wire Unipolar steppers, and as you can see from the wiring digrams above you can just not connect the centre taps for the coils (labeled A and B above) and we have a bipolar stepper again.
More recently when I decided it was cheaper and simpler to use two steppers for my Z-Axis, I was a stepper short. This time I bought the SY42STH47-1684B compatible product from Zapp Automation for only £8.50 it has a holding torque of 43.15N-cm and is a 4-Wire Bipolar stepper. I am going to use this to drive the CrudeStruder, as I think it could benefit from the extra torque.
Giles Bathgate 
This is kinda what makes steppers a pain in the butt, I have 2 smaller ones I would like to use but they are the 5 wire unipolar ones. I still do need to find another stepper motor though.
I don’t know how you could wire up a 5-wire unipolar stepper to work with a Stepper Driver 2.3 but there is a simple circuit here that might be suitable depending on your load requirements http://arduino.cc/en/Reference/StepperUnipolarCircuit
You may have one problem, but on the positive side you can use much cheaper driver circuits for unipolar motors. In addition you can make a much cheaper circuit that is able to supply higher currents to the motors than many driver circuits you get for bipolar motors.
Just to clarify, the Stepper Driver 2.3 does work with the FL42STH38-1206A Tried and Tested. (Leave center taps for the coils unconnected) However if your stepper is a 5-Wire Unipolar stepper, your mileage may vary 😉
For wiring up the extruder controller 1A = Black, 1B = Green, 2A = Red, 2B = Blue
For wiring up the stepper drivers A = Black, B = Green, C = Red, D = Blue