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

Small Business Create Visio-like Structured Diagrams for Free By Dave Johnson Updated on: October 15, 2009 / 11:57 PM EDT / MoneyWatch ...

HVAC Controls – A simple heating, ventilation, and air-conditioning control diagram. Go to Microsoft Subnet for more news, blogs, opinion. More Microsoft Subnet blog posts ...

That said, I don't especially care for Visio; it's unnecessarily complicated, and it costs money. Now, though, I can use Project Draw, a free Web app from Autodesk.

Using templates is the easiest way to begin making diagrams in Visio. I’ll walk you through three of the most common Microsoft Visio use cases — creating flowcharts, org charts, and Venn diagrams.