Dia :Tools to draw more than 30 different diagram types ( database models,network diagrams,flowcharts....)

Dia Diagram Editor

A great tool , a superb tool

Popular general purpose drawing software for Windows, Mac OS X and Linux. Dia supports more than 30 different diagram types like flowcharts, network diagrams, database models. 

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EXAMPLES : 

>UML Diagram : 

>DB Diagram :