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.

mxgraph is another open source, pure JavaScript, client-side library for creating interactive diagrams that renders to SVG and HTML and it’s free! The authors explain: ...

Microsoft to open source Chakra, the JavaScript heart of its ... 2015 9:40 am | 77 Block diagram of Chakra's design ... ChakraCore cross-platform—just as it did when making .NET open source.