We've shared a lot of ways to automate the lights, doors and furniture of your home, but DIY weblog Mad Science has one of the cheapest, simplest home automation projects yet: turn anything on or off from afar with an old mobile phone and an Arduino.
All you need for this project is an old mobile phone (with either Wi-Fi connectivity or prepaid SIM) and an Arduino (along with a few other assorted components for the Arduino). You'll need to know how to get up and running with an Arduino and someone else's code, of course, but the project is very simple: essentially, the Arduino has a light sensor attached that will detect when the mobile phone lights up (like when it receives a phone call) and will perform an action in response, like turning on the lights. Throw the whole thing in a box, stick it in a corner, and you have a mini home automation project set up for anything you can think of. There are certainly more sophisticated ways of doing something like this, but it's a great way to get started with projects in this realm, and it's very cheap if done right. Hit the link to check out the full how-to.