Controling a regular residential 110V/15A circuit switch (or relay) from a PC
There are a number of home automation solutions that may meet your needs.  Current technologies include both PowerLine Communication (PLC) and RF.

They typically consist of a relay or Triac-based controller that mounts in the place of a traditional light switch.  There are appliance modules that plug between outlets and appliances (and some that replace the actual receptacle themselves).



There are also PC-based systems that interface via USB and transmit/receive the necessary PLC/RF to support full-duplex communication with each of the individual switch modules.

These systems are nice because all of the modules that touch line voltage are ETL recognized (superset of UL). This mitigates the safety and insurance liability aspects of a homebrew solution, though you should still have a qualified electrician install them.

I believe there are also thermostat modules in the same line, though I have no experience with them.

They're sold under the brand names X-10 and Insteon to name a couple, though there are others.