3d sketching

I’ve got an idea drawn out for a piece of furniture I want built – and will probably end up building myself – but I’d like to do it digitally to see if I really like it. Does anyone know of a good open-source/freeware/demo 3d/CAD program I could use to achieve this? OS doesn’t matter since I now have Linux, Mac, and Windows in the house. You guys always pull through for me – leave something in the comments…