Apple - What are Mac OS X replacements for Visio?

OmniGraffle or OmniGraffle Pro: http://www.omnigroup.com/products/omnigraffle/feature_comparison/

As others have mentioned OmniGraffle is a great Visio replacement. But if you want something free, I'd also recommend yED:

http://www.yworks.com/en/products_yed_about.html

I use it on my Linux-based systems, but since its java-based, it works well on Mac OS X, as well as Windows.

We produce diagrams.net, it's not as complex as the named tool, but it does the majority of tasks in this field. It's free, plus there's a Desktop version available.