I am using snow leopard and mainly want it to automate the stacking as much as possible. I have seen that PixInSight looks pretty awesome, but that's not free, in-fact far from it, so I want something that is free that can do the same, or as much as possible.
Hugin might be your answer. At least it is available for the Mac and handles stacking of images (also into EXR-format), masking and distortion-correction of images. It is not per se meant for astrophotography, so you would have to know what to do.
As for automation (of the aligning) - I put each of my Stacks/Stitches into separate directories and just let a script run over all these directories, applying "autopano" or "align_image_stack" on all files in the directory. You have to check and correct the ready .pto-file afterwards.
I've been using StarStaX to stack photos to make star trails. It's features are relatively limited in that it really is specifically for stacking images to create star trails only, but it's easy to use and works well.
CombineZP should run fine on an Intel Mac (after you get rid of OS/X and install Windows).
Edit: Since a lot of people seem to have missed the point and think this isn't helpful, let me try to put things more directly. I am reasonably certain there is not currently any free (in either sense) software for stacking that runs directly on MacOS and is anywhere close to as good as CombineZP.
I'm relatively OS-agnostic -- I have two Macs (a 24" iMac and a now rather elderly Macbook). For me, choosing between Windows and MacOS is mostly a matter of picking what works better. For stacking, I use CombineZP on a PC. I have not been able to find anything that runs directly on MacOS that produces nearly as good of results, or is nearly as easy to use (for this task). Hugin works quite nicely for stitching panoramas, but stacking isn't really its forte. It can sort of work, but it's semi-painful to use for the job (all the docs and UI are oriented toward stitching panoramas) and even with quite a bit of manual labor, the results from it are still inferior to what CombineZP produces entirely automatically.
Bottom line: A copy of Windows + CombineZP costs less and works better than anything you can get natively for MacOS. Sorry, but that's just how it is.