Direct SIO Connection (à la APE, SIO2PC, MULE)
Presently, about the only way to transfer files between the Mac and an Atari is with a null-modem cable. This works, but is limited and slow.
PC owners are fortunate to have a better alternative. A number of years ago, a guy named Nick Kennedy designed a special cable which goes between the Atari SIO (drive) port and the RS-232 port on a PC. Utilizing the cable, software such as SIO2PC and APE allow the PC's hard disk and other peripherals to be used by an Atari. The files can be managed on the PC side and the Atari can even boot from ATR disk images on the PC. Steven Tucker's APE goes way beyond the basics and even allows transfer and running of copy-protected disks. (It's also the only one still actively supported and upgraded.) MULE uses the cable for simple file transfer.
There is no technical reason such a system could not be created for Macs, but so far, no one has done it (though discussions show up on the comp.sys.atari.8bit news group occasionally).
It may be possible to modify the cable to fit one of the Mac's serial ports and use APE or SIO2PC with a PC emulator (Virtual PC or SoftPC) or co-processor (DOS) card. One problem with this approach is that the Mac's serial ports are RS-422 which, while similar and compatible to the PC's RS-232 in most ways, are different enough to make such a work around problematic. Specifically, the Mac's serial ports lack a "ring detect" line. The APE software does have an option not to use this line (for special cases having to do with running it on Windows), but I haven't tried it. I suspect it might work with a DOS card, but it seems less likely with an emulator since the APE and SIO2PC software appears to have very stringent timing requirements.
There is one exception--the OrangePC cards, which have their own RS-232 ports. Unfortunately, these cards are no longer available and I've never seen a used one for sale. For much less than that you could get a complete used PC system (even an otherwise worthless model such as a 286 can do the job), but then you'd have another box to deal with. This is the route I have decided to take. I got a discarded 486 PC from work, but I haven't gotten around to getting the cable yet. But it would be much more convenient (and take up a lot less space) if there was something like it on the Mac.
Another possibility with newer Macs is using a USB serial port adapter, but I have not looked into this yet.
If you know of anyone who is working on something like APE or SIO2PC for the Mac, drop me a line.
Mac-based 6502 Cross-assembler
At present there are several PC and UNIX 6502 cross-assemblers. I have not come across one yet for the Mac. If you know of one, write to me and I will put a link for it here.