Not everyone has the money to spend on a professional diagram editor like Visio, but there are some free solutions available as well. Many of the free options are available online as web-based editors, but if you need a desktop app something like the Dia Diagram Editor may be what you want. It isn’t as full features as Visio, but it does have a lot of object libraries available meaning you should be able to accurately represent everything in your diagram.
What kind of libraries does this include? There are libraries for circuits, computers/Cisco devices, databases, electric, flowcharts, lights, logic, network, UML, and many more. The quality of the graphics may be disputable, but keep in mind that you’re not paying a dime for the app. There is something to be said about that.
This is one of the more powerful free diagram editors out there, and the fact that it is cross-platform doesn’t hurt either. So if this sounds like something you’ve been looking for go ahead and jump over to the homepage.
Dia Diagram Editor Homepage (Windows/Mac/Linux; Freeware)