Recently, Apple has endorsed a port of Linux to the PowerPC Macs. Check it out at:
In addition, there is an ongoing MacLinux/68k effort. For more information, see:
There is no current compatibility with MacMiNT.
More information on MacMiNT is available at Hoshi's MacMiNT Page.
Unfortunately, MacMach died out years ago. However, the PowerMac Linux project is built on a version of the Mach microkernel. Check out Apple's Apple and the Open Software Foundation Announce Linux for Power Machintosh press release from February 1996.
You can mount A/UX filesystems read-only. There may be binary compatibility with A/UX binaries in the future (don't hold your breath, though).
You can use Lawrence Kesteloot's hfs tool to access Mac OS filesystems as read-only. Use this tool to browse the filesystem and copy files to a NetBSD/mac68k partition. Someday we hope to run Mac OS as a process on top of NetBSD/mac68k.