Zemulator is an application that allows you to emulate a ZPL compatible printer. While running, the application will listen for print requests on port 9100. If your computer supports Bluetooth Low Energy in Peripheral mode, it will also act like a bluetooth ZPL printer.
Labels are rendered via the wonderful Labelary folks and displayed in the application window.
You can adjust the emulated details of the printer, such as label size, in the settings panel.