3d sketching

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