Controller for HAI® Omnistat™ RC-Series Communicating Thermostats
I wrote a small software utility to control the thermostat at my house from a PC. This makes it possible to adjust the temperature from a laptop anywhere in the house, or even from the other side of the world. The thermostat is an HAI Omnistat RC-80B.
Typically the thermostat will be connected to a serial port of your home server. The software also supports accessing the thermostat over a network; to use this functionality you'll a device server (for example by MOXA) in order to get the thermostat online.
This was written on FreeBSD, and has been used on Linux and Mac OS X as well.