OmniGraffle
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OmniGraffle is a diagramming application made by The Omni Group. According to the site, OmniGraffle is built specifically — and exclusively — for Mac OS X.
In many respects, OmniGraffle is similar to Microsoft Visio. It may be used to create simple diagrams, flow charts and illustrations. It features a drag-and-drop WYSIWYG interface.