As part of my Silvia modifications, I also wrote a special application to help in tuning the PID. Here is a screenshot of the application with an image of a shot and recovery. This is measure the temperature at the top of the boiler.
The application is written with Processing. It can run on any platform, and it's easy to communicate with the Arduino over the usb-serial interface. The "Bare Bones" code for the Arduino is here and that page has a link to the code for the Processing Ap.