Insteon is a home automation technology.  It was designed after X10, and meant to replace X10, but many insteon products can also accept x10 messaging.  The primary difference between X10 and insteon is in the reliability of message transmission.  Never having worked with x10, I understand that it works something like e-mail -- you send a message out, and just have to trust that it got there.  Insteon, on the other hand, can incorporate ACK/NAK replies and all insteon products will rebroadcast insteon messages so that messages have a higher probability of reaching their target.

Insteon was developed by SmartLabs Technology, a division of SmartLabs, Inc., a SmartLabs, Inc. company, retails insteon products along with other items.

Insteon has a variety of products, but the primary ones I've found useful as I begin to utilize this technology are switches, outlets and relay controls.  The swtiches (dimming for lights, non-dimming for fans) allow fixture control, and the relays allow me to control irrigation valves (and can enable sense of items like alarm contacts).  With Insteon, I can turn lights and irrigation on/off from a computer.  I can have the computer check to see if I left the garage door open.  Insteon also has a device that can enable remote control of a thermostat!

I'm registered as an Insteon developer.  To become registered, I simply had to buy a developer kit.  SmartHome will not register you as a developer without purchasing the kit -- even if you buy a PLM and other components included in the kit.  The big thing you get as a registered developer is access to the developer site, which includes some great documentation on the insteon protocol.

Using a PLM, I have almost raw access to the insteon network established across my powerlines.  I don't have raw access because SmartLabs built security into the PLM, making it only relay messages from devices I've linked it with.  Until I've linked the PLM to another device, I won't receive messages from that other device.  This means if my neighbor got a PLM and plugged into one of my outside outlets, he wouldn't be able to manipulate my insteon network because he wouldn't know the addresses of my devices, and since he wouldn't know the addresses of my devices, he couldn't link with them (and therefore wouldn't see any insteon traffic on my power lines).  Moreover, there is a protocol for communicating with a PLM, which serves to encapsulate the insteon protocol.  I must know and understand both protocols to make use of the PLM.

Let me show a quick video to illustrate relay control over the power line via insteon.  I send a command through the PLM to the EZIO in the video.  It opens the relay (and thus opens the irrigation valve).  The computer then waits a little while and closes the valve for me.  The EZIO can include fault mitigation in that it can be configured to close relays automatically after a variable period of time (either minutes or seconds, ranging from 0-255).

Insteon EZIO Relay Control