GIS on the Macintosh
Command line driven
OpenOSX specializes in porting UNIX software to the Mac. They also have Wintel, a virtual machine for running Windows and Linux on the Mac. See Virtual machines on the Macintosh
Looks nice but so far I can't get it to open shapefiles, this could be a serious drawback.
http://www.orbitgis.com/ (Orbit GIS)
Low cost (EU299) enterprise program, free Orbit Explorer viewer
Support for Mac OS X, Windows, Linux
Links I have not followed up on yet.
http://qgis.org/ (Quantum GIS)