How can I use my computer to control my house?

http://www.instructables.com/id/Control-lights-in-your-house-with-your-computer/

Here is a tutorial on controling lights with a computer.

http://www.gadgetmadness.com/archives/20040320-computer_control_your_house.php

Web store dedicated to home automation products: http://www.smarthome.com/

One other product solution that mentions light and appliance control:

http://www.x10.com/products/x10_ck11a.htm

X10 is a costy professional switching system, with quite limited capabilities (no automatic cat feeders or a cat cam for example).

So you might consider playing with Arduino www.arduino.cc This is a small stand-alone "computer" that can be programmed with ease over USB. The nice thing is that Arduino does not use much power compared to running a full PC. It is so small, you can hide it wherever you like.

From the basic Arduino, you can add several shields to switch appliances. There is a relay shield for 110/220V switching You can make Power-Mosfet shields for high frequency 110/220V switching You can add an XBee shield for wireless control of other XBee switches. There is a big bunch of input shields to read RFID, measure temperature, measure light, sample sound etc.

Pricing is great. Arduino costs around 20$, most shields do a similar price, and if you can solder you can cut on costs even further.

A good source for arduino projects is: www.makezine.com They have a nice store too: http://www.makershed.com/SearchResults.asp?Cat=43

Good books on the subject are "Making things talk" and "Getting started with Arduino" If you buy the Arduino starter kit, you have a great set of components to start with automation, and the nice book "Making things talk" is included as well.