- Most SCH tools (see above).
- Microsoft Visio -- part of the Office suite (but not the main package, you have to get it separately I think).
- Most 3D CAD e.g. SolidWorks. Can include physical routing, if you are constructing a complete 3D model of the product. If nothing else, 2D drawing output.
- Any 2D CAD, e.g. AutoCAD. Harnesses and wiring diagrams are lightly abstracted in a schematic form so all you need are the shapes, line types and connections.
Downside of SCH tools: you may be stuck with either plain 2D drawing (and usually fairly limited at that, no plane geometry functions?), or abusing SCH objects (symbols, net labels, etc.) for a purpose not quite the same as what was intended. In which case, you may have to grapple with ERC (electrical rule check) or name collisions or connectivity. Upside, you can get a netlist output from the graphical connectivity -- but this may be of limited usefulness.
Downside of most others: doing SCH-like operations (drawing blocks, and connections between them) is kind of special-purpose, so you end up taking a lot of time making groups/objects/symbols and drawing the lines exactly between them, rather than concentrating on structure and presentation.
Flowchart tools are not to be overlooked. If you can do manual placement and routing, and custom symbols, that's pretty much all you need. Visio is pretty commonly used in this way, and isn't bad at all. I don't know of others offhand, but I think there are several at least?
Tim