What are Mac OS X diagramming tools like Visio on Windows?

Visio is the most conspicuous example of a program that I found useful on Windows for which I have not identified a Mac-compatible equivalent.

Visio's primary function is as a flowchart diagramming tool. It allows the user to construct complex information flows with smart objects and interconnect graphics that snap and bend as the user moves things around on the page. It evolved over the years to support other diagram and information flow views like IT diagrams and UI mockups.

What's out there that can replace Visio on the Mac?

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.