From Mark Grebe:
Sio2OSX is a peripheral emulator for the Atari 8-bit computers that allows the Atari computer to use an OSX based Macintosh as a disk drive, a cassette drive, and a printer. Sio2OSX peforms functions similar to APE or SIO2PC on Windows based computers. Sio2OSX is a shareware program, and must be registered for full operation. The unregistered version of the program will run for 5 minutes before quiting, allowing you to verify operation with your hardware setup before purchase. The software can be easily registered for $25 through Kagi. A full manual is included within the application's Help menu. Note, as this does require serial USB hardware, and is a new product, please try it out on your hardware before you pay the registration fee. I'll be glad to answer any questions you might have by email.