An evolving Arduino PID controller for espresso machines. Successfully tested on Rancilio Silvia http://arduinoespresso.blogspot.com/
http://ukrduino.blogspot.com/2010/01/end.html По Русски!!!
There is a lot out there: google any combination of "mods modding coffee pid home roasting espresso machine silvia" (please add other useful) and find a ton of great stuff.
Solid State Relays There are many out there, and it can be difficult. I used to like ebay, but I find it difficult at times to sort through all the wrong kinds. I've also accidently ended buying the wrong ones too!
Here are some nice surplus ones for $9 (april 08): Item CY3303 http://www.surplussales.com/Relays/RESSPlug-1.html (update: I actually don't really like these (just rxd them in the mail). They are cheap, and work, but don't come with screws, or wiring labels, or anything. I would instead splurge on some nicer ones in better condition. The best have LEDs that turn on when they are switched on)
Temperature Sensors Two easy ways to get temperature are with devices that can be plugged into the arduino's analog inputs and provide a voltage with a linear correlation to temperature. One is a thermocouple and an AD959, both available at Sparkfun. The other is to use an LM34 (Fahrenheit) or LM35(Celsius) from National. These can be had at digikey. There are two basic packages. One looks like a standard black transistor (TO-92), is good to 250F, and is about 6 bucks (http://www.digikey.com/product-search/en?pv1291=714&FV=fff4001e%2Cfff80137&k=LM34). The other is a metal package (TO-46) and is good to 300F, and is about $14 (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=LM34AH-ND).
Display There are a bunch of options around for this. Choosing a display that allows serial control will make life much easier for you. If you are adding a lot of features, it might even be necessary as you may start running out of program memory with a display requiring more control code. This one, or any other color option: http://www.sparkfun.com/commerce/product_info.php?products_id=813
Real Time Clock http://www.sparkfun.com/commerce/product_info.php?products_id=99 This little board from sparkfun is nice because it includes a backup battery, and crystal required by the clock.
Water level sensing
The capacitive sensor IC (which you might not even need as Arduino can do capacitive sensing right off the board...) is a
QT114 QT113 from Quantum. The trick to use it for water level sensing is to set it to never time out when its sensing the water (from datasheet: set pin3 to GND, pin4 to Vdd). That is available at Saelig.
Here is a tutorial for it. and the
Then for the sensing electrode, get an insulated wire with a cm or so left unshielded at the end, and put that down into the water reservoir. When that unshielded tip goes dry, it will trigger the sensor. Alternatively, it is possible to make a sensing electrode that is external to the water tank, using "de-soldering" braid glued to the tank. This requires careful selection of the reference capacitance, Cs. When , installed this is one way to set it up.