ZonCoffee is full-featured PID controller package for Arduino and customized for use with espresso machines. It can be used to display espresso machines set point and current temperature information on LCD. You can adjust espresso machines temperature easily via rotary encoder. Other feature is steam and extraction modes with separate temperature targets. The project comes with Intuitive serial interface which compatible with the BBBC grapher. Simple configuration can be done in Options.h file.
Ethan said that the entire project can be built for around $40 (not including an Arduino) assuming you acquire a free sample of the MAX6675 chip. The sketch size is around 14kB compiled, therefore, an ATMega168 or higher is required for this project.