I have used some software called 'target' in the past. It is fairly basic, and easy to use, but is limited. Programs like turbocad, autocad, etc all have shape libraries suitable for doing wiring diagrams. I use autocad for simple diagrams.
Do a google for 'free circuit design software', or 'free pcb design software'.
The pcb software gives the ability to make printed circuit boards, which I know your not worried about. They should also include the ability for schematic capture though, which is the name used for circuit design.
Hope that helps.
Cheers,
John.