This can be useful in a lot of different circumstances and it's a pretty simple mod. Me personally, I'm building a Prusa i3 and don't want to have it powered on all of the time, but it's so much more convenient to just leave it plugged into the back of my desktop since it has it's own dedicated power supply.
Knife (a sharp box knife will make you're life a lot easier)
Scissors (this will help a lot in removing the insulation)
Step 1: Step 1: Strip the Insulation
First things first, you need to strip the outer jacket of the USB cable. I did about a 1 inch section which allowed me more than enough room to cut out the necessary cable. Also, I made two circular cuts in the insulation and one cut connecting them. This allowed me to reuse the insulation when rewraping the cable for added protection.The one I moded is the kind that you'll commonly find with Arduinos and they have a wire mesh and foil coating around them as well. I cut through both of them (as you can see) and the cable still works just fine.
Step 2: Step 2: Cut the Necessary Wire
For USB Type B cables, this is the most common color coding for the wires you'll find inside.
Pin 1 - RED: VCC
Pin 2 - WHITE: D-
Pin 3 - GREEN: D+
Pin 4 - BLACK: Ground
Pin 1, or the VCC cable is the 5V power supply cable, this is the one that you'll want to cut if you want to have a Data Com only cable. YOU MUST LEAVE THE BLACK OR GROUND, it is required for the Data Communications and if you cut it, this will NOT work. Conversely, if you'd like a power supply only cable, you can cut the White and Green cables.
Step 3: Step 3: Rewrap the Wires
This step is pretty simple. Just wrap the wires in electrical tape and you're done! I saved the outer insulation from when we cut it off and used it to provide a little more rigidity.