ZigBee

Comprehensive standard (IEEE-ratified) for home automation, energy management, etc. Unfortunately, the "standard" does not appear to let devices be interoperable with equipment from different vendors.

Getting ZigBee equipment seems difficult: you have to resort to eBay, or deal with dealers with extremely high markups. Cannot order equipment cheaply or easily.

Largest vendor in the space appears o e Control4.

Z-Wave

Controllers

Mi Casa Verde Vera 2

Runs Linux and OpenWrt? Free (no monthly fees). Web UI, accessible through company's website.

Linux Journal has a review of the Mi Casa Verde 1. Yes, that is a bash script controlling the lights.

Homepage. Amazon

TrickleStar 300ZW Z-Wave Dongle

Cheap. Appears to work w/ Linux. Amazon

Devices

GE On/Off switch. Amazon.

Trane thermostat. Amazon.

Software

open-zwave. C++ library for interacting with Z-Wave dongles/etc… seems very basic still.

Stores

HomeSeer has a large selection.