Sidchip made an adapter that allows connecting two joysticks to USB. The features are: Two digital joysticks simultaneously, Supports for paddles on port 1, Compatible with Windows, Linux and Mac. The device is built with a PIC18F14K50 micro controller, the schematics, PCB layout and firmware are available.
Website: http://kair.us
This is so cool. Emulation is getting closer to the real thing all the time.
There is something rather cool about using a zipstik on my Mac!