Please note: These instructions were accurate for the old Raspberry Pi boards and the older version of Raspbian, but there have been big changes recently which require new steps, at least on the software side. The up-to-date instructions (kernel > 3.18, Raspbian as of Feb 2015) are on ispresso.net.
iSPRESSO is an appliance modification comprised of Raspberry Pi computer, solid state relays, temp sensor, buttons and a display, a custom Printed Circuit Board, and custom linux shell scripts and a good bit of python code. The instructions to build, including source code, is available on this site and ispresso.net
WARNING: Before you get started, be aware that messing with electricity near water is very dangerous! Be sure you know what you are doing, take every precaution, consult an electrician, and be 100% sure you are using GFCI outlet.
I've put together a list of most of the items you will need to build this. To the extent possible, these are Amazon Prime items: