So to post a formal answer-
The program I used to create the diagram in the linked post was Adobe Illustrator, which is definitely not free.
That being said, any descent vector drawing program can easily make diagrams like these- one free one that comes to mind is InkScape. That combined with a good library of commonly used symbols in SVG format would make creating these diagrams much easier. I have been able to find plenty of typical electrical circuit symbols but have yet to find home wiring symbols. I have several that I have made, that I'd be happy to release if they would be useful.
Edit- another program I have used in the past that is free is xfig, which is a simple vector drawing program for X11 systems. Works great on Linux or OS X, and it includes a pretty big library of schematic symbols.