Being a software student I have to usually make Flowcharts, DFDs and ERDs. Windows people have Office Visio as part of the Office suite for all their diagramming needs but there is no Visio in Office for Mac.

OmniGraffle is an excellent diagramming, charting, and visualization software for Mac OS X. I use this software for all my diagramming needs and find it very easy and flexible to use. Omni Group also gives educational discounts on all their software to the qualifying people through their edu-store.