Dia is an open source diagramming tool similar to MS Visio. It has been stable with only minor developments for a few years, but it is a useful tool for simple 2-D circuit diagrams for discussion and documentation.

Custom shapes for analog and digital schematics and timing diagrams
A set of custom shapes I’ve developed is maintained in a public repo:
https://github.com/shumphreys2/SRH_circuit_shapes

Here’s an example schematic:
example schematic

More about Dia
Documentation and prebuilt versions for Windows, MacOS and Linux are available at the links below.