A Raspberry Pi Remote Mains Switcher, to switch these

http://www.maplin.co.uk/remote-controlled-mains-sockets-5-pack-348217

with one of these

http://proto-pic.co.uk/434mhz-rf-link-transmitter/

using the awesome reverse engineering detailed here

http://www.fanjita.org/serendipity/archives/53-Interfacing-with-radio-controlled-mains-sockets-part-2.html

Requires WiringPi-Python

git clone https://github.com/WiringPi/WiringPi-Python.git
cd WiringPi-Python/
git submodule update --init
sudo apt-get install python2.7-dev python-setuptools
sudo python setup.py install

You'll need something like this http://proto-pic.co.uk/434mhz-rf-link-transmitter/

Assuming that is the one you buy, the transmitter has 4 pins. Wire

If you've had to read this section, please see the disclaimer below.

Needs to be run as root -

sudo ./strogonanoff_sender.py --channel 1 --button 3 --gpio 0 on 

where the GPIO pin number is 0 (the default) if wired as per the table above, or otherwise the big ones on this diagram

It works for me, but connecting stuff to your Raspberry Pi can destroy either end. If it does, no matter how negligent I've been, I'm sorry, but it's your problem.