Ethernet over power (as opposed to power over ethernet)
Specifically, the D-Link DHP-300. You get two of them, you plug them into the wall, and you plug an ethernet cable into each one, and it's sort of like a big, long ethernet cable.
After considering fancy-pants wireless extender antennas, I decided to go with these in order to connect the two parts of what you might call my "disjoint apartment," which consists of some rooms that are just not reachable from other rooms. Imagine a labyrinth in which you can't actually get from the entrance to the exit. And you're an electron. ... It's a long story.
A quick review: well, in short, everything they say here is true. It's sure as heck not getting the "theoretical" 200 MBits. In my apartment, which admittedly has some very bad ancient wiring, I get somewhere around 8 MBits when it's in a good mood, which is sometimes.
Performance depends on which outlet you plug it into. Some don't work at all, probably due to split-phase related problems. Oddly, the extra outlet on my stove works best (yes, I tried them all). But somehow that just doesn't seem like a very good idea.
I've seen a few comments to this effect, but I can absolutely confirm: DO NOT use a power bar, surge protector, or even one of those multi-outlet plugs with these things. The signal quality degrades dramatically. Plug it straight into the wall, and save your power bar for other stuff.
Of course, plugging my wireless router into the other plug on the same outlet also greatly degraded my signal, which is rather inconvenient. I had to plug it into another outlet across the room. Sigh.
Also, turning the room lights off make the signal worse. But only temporarily.
So I suppose the real story here is that maybe I should have just bought the new and improved wireless antenna after all. All that said, though, it's somewhat amazing that it works in the first place. They deserve some credit for that. And since all I need it to do is cover my 6 MBit cable internet connection, it's actually just fine. Even Skype (with video) works perfectly, with no extra lag.
This sort of thing is doomed to remain a niche product, though, as long as it requires so much fiddling.