In London's Docklands, at our rented house we could see more than 23 different wifi networks crowded around us. We had to spend some time to find the least congested channel to get any usable signal for even the most basic web browsing. The minute some neighbour came home with a new wifi router and plugged it in, it threw off our carefully balanced setup, and we'd have to run through the channels again.
As it was a rented house, running Cat5 was not an option. We bought Devolo Homeplug AV bricks and I was very impressed by the true plug and play nature of them. Streaming video from the servers upstairs to the Mac Mini under the TV worked perfectly.
I'm very glad to see that products now include multiple ports and even electrical outlet replicators. It was very annoying to have to use a Homeplug brick and then a separate switch in order to serve multiple networked devices.