From canuck:

Clarification on what kind of data visualization features are being sought would definitely be helpful. I don't know from Mac, but Google and Many Eyes are the main alternatives to Tableau, though each have irritating limitations in their way.

If their interest is mapping, they might look at Ushahidi's Crowdmap.

If they know what they're doing or have some time to learn, they could look at Protovis, but that has more of a learning curve and requires other types of resources in terms of sharing data online and collaboration.