A list of free Microsoft Visio alternative software to prepare complex diagrams includes Kivio, Graphviz, Inkspace, Open Office Draw, and DIA Diagram Editor.

QElectroTech is an open source tool for drawing these types of diagrams. It consists of two tools: an element editor for creating schematic symbols and a diagram editor for creating your drawings.

Dia (Figure B) is an open source, GTK+ diagramming tool that has a shallow learning curve and can help you create basic network diagrams. Like CADE, Dia was inspired by Visio — but with a much ...