olivernz: Ethernet I'd say run a cable next to every power outlet even if you don't use them.

I definitely wouldn't do that, you're going to get all manner of mains voltages and noise coupled onto the Ethernet cables if you run them next to mains wiring.

This can also lead to interesting results if you're using a high-impedance multimeter to check for the presence of voltages on wiring, a high-impedance meter will report the presence of ghost voltages coupled across from other lines, which is why electrician's meters have low-Z measurements that will detect and load the line sufficiently to give an actual reading.