If its simple block diagram or a generic placement/layout guideline for PCB, I use inkscape. Aligning & resizing objects is very easy in inkscape.
If you want to display flow control i.e. arrows, I will suggest dia or draw_io. Only thing that bugs me is the occasional odd snapping of arrow pointers in these 2 tools.
Dia has an extension 'diashapes', which consists ready drawings/logos categorized under multiple topics e.g. switches or relays if you want to draw an electrical wiring diagram. Even logic gates & more.