Grass 5, the open-source GIS software, is available for MacOS X. I’m tempted to try installing it again, but it requires installing a number of components, like a database server and x-windows clients. I went through the process last fall, but realized I didn’t know enough about Unix or databases to continue.
I know enough about Unix now, but still haven’t done any database stuff. I suppose reading is a good place to start, and some clever people have created an installer CD for OS X. And now there’s a book available, though its $119. Its still cheaper than buying a PC and ArcView GIS…maybe.