I am using an ULN2003 Darlington arry http://arduino.cc/en/Reference/StepperUnipolarCircuit to drive a unipolar stepper. Would you please advise what diode I should use at com? Thanks.

 

Have you downloaded a datasheet for a ULN2003?

If you will look at one, you will find that each of the seven Darlington channels has it's own protection diode, and pin 9 is the common connection for the cathodes.

It will be fine just connected like in the schematic.