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.