Insteon
I've always been into cool lighting. I wired my old apartment for X10, and had a wireless key fob that I'd use to turn all the lights off as I was leaving; cool and useful.
X10 is a protocol developed in 1975 for controlling devices via the power line. The Wikipedia entry for X10 covers it pretty well, including some of the limitations of X10.
With X10, you manually assign an address to each device, and then send commands like "Device A3, Turn On". Any device A3 that receives that command will turn on.
Problem with having X10 in an apartment building is you're sharing the power wire with other apartments, and I actually ran into a namespace conflict with someone else in the building - for a while, my lights were randomly turning on and off, and presumably when I sent commands to my devices, theirs were acting up as well. Switching house codes fixed that, but it's a clear limit of X10.
The original X10 protocol provides no way for devices to communicate their status or acknowedge commands - so you'd ask A3 to turn on, cross your fingers, and assume it happened.
The Insteon protocol was developed by one of the major manufacturers of X10 products, SmartLabs, to solve a lot of the problems of X10. Among the improvements are that devices have a 24 bit preassigned address (much like a MAC address), every device is a repeater (so signals are much more likely to arrive at their destination), and that a lot of the features added to X10 over the years are standardized by the Insteon protocol, so consistently supported across devices.
If you're not familiar with Insteon and at all interested, read this What Is Insteon page.
Anyway, back to my experience. When I moved from the apartment to a house, I found out that I didn't have the necessary wiring for two-way X10 communications. A neutral wire at the switch is required for this, and unless you specified this when building your house, or unless you're just lucky, you probably don't have it.
When we bought the house we're in now, I specifically asked for the neutral wire to be at every switch. It took quite a few tries to get the builder to understand what I wanted, but eventually, after talking to their electrician, they agreed, charging me ~$650 to do this for every switch in the house. Sounds expensive to me, but I didn't have any choice, and it's something I wanted, so I went for it.
I ordered the Insteon Starter Kit, and it was delivered Friday. I spent a few hours today setting it up, and now I'm happy to say, it works great!
I learned a few things in the process... like that the colour of the wires in the switch doesn't mean anything. Silly me, I thought there'd be standards for that. But with the help of a SmartHome support guy and a $15 tool from Home Dept, I figured out the wiring part.
It just works. It took a while to get everything configured and all the switches linked up to the keypad, but now that it's done, I can turn on any of the lights and the corresponding button on the keypad lights up, and I can turn the linked switches on and off from the keypad.
It turned out this became a necessity in our new house anyway, because some of the switch locations just didn't make sense and we didn't pay close enough attention to the wiring diagram to catch it before we moved in, and by then it was too late. Now that the new keypad is in, we don't have to walk to the back of the house to turn off one of the kitchen lights anymore before going upstairs.
Anyway, all this to say, Insteon seems to be the future of powerline based home automation, and I'm looking forward to new products coming out next year as it starts to really take off.
One caveat to the Insteon Starter Kit is that the switches that it comes with don't like compact fluorescent lamps, so if you've done that "replace all my bulbs with CF bulbs" thing, you'll have to buy a few incandescent bulbs for the lights you're controlling with the new switches.