I’ve been using a really clever device for the last few years that a lot of people seem to be unaware exists. It’s an ethernet over powerlines adapter – one such example is the Devolo dLAN. In a nutshell, you plug it into the mains, and connect the Ethernet socket to your network device. You can then plug as many others as you like to various other powerpoints and extend your network wherever the power stretches. Devolo do ones that run up to 200mbit. It’s a theoretical maximum, although I’ve got 177mbit before which is impressive. It has a couple of downsides:

1. It won’t traverse 3 phase power. I’ve tried it, and I’ve ended up with a very weak/nonexistant signal which is probably more inductance than anything else.
2. Obviously it doesn’t handle bad cables well – it doesn’t much like extension cables either.
3. Different circuits work about as well as 3 phase power, the only signal you will get is probably inductance between the two circuits.

Some advantages:
1. It travels pretty far. I’ve had over 150mbit between adapters at opposite ends of the house.
2. No new cabling
3. Fully supports standard Ethernet so all network protocols will work just fine over it.
4. I love it

For anyone running a home or office network and not fortunate to have Ethernet points cabled in, I strongly recommend these devices, you’ll never know the difference.