The PiFace Relay Plus module.

http://pifacerelayplus.readthedocs.org/

You can also find the documentation and some examples installed at:

/usr/share/doc/python3-pifacerelayplus/

Try this first:

sudo apt-get update
sudo apt-get install python{,3}-pifacerelayplus

Download the debian package from here and install with:

$ dpkg -i python3-pifacerelayplus-0.2.0-1_all.deb

You may need to download and install the latest release of pifacecommon and reboot.