Most common tools I hear people mention for wireframing on the Mac are OmniGraffle, Illustrator, and InDesign. I lean towards OmniGraffle because it's fast and easy to use for creating interface schematics with any of the UI stencils avaialble including my own, is a very capable as a drawing program, allows you to do multi-page documents, and quick and dirty HTML or PDF prototypes.
I use Illustrator when I do visual design, but OmniGraffle for wireframing. And to tell you the truth, I have found myself doing very high fidelity wireframes with OmniGraffle occasionally after many iterations back and forth with designers who work on the visual design.
Axure is an excellent program, by the way. I used early versions for about a year when I was on a PC and the company I worked for paid for it. It's on the expensive side, which is why a lot of PC folks stick with Visio.