The osPID is an open source modular PID controller. PID is used to monitor a process and provide feedback, such as the temperature controller on a DIY reflow over or sous-vide cooker.
The main board has an Arduino and a user interface panel with an LCD and a few buttons. Two daughter boards are used to get different input and outs for the PID control loop.
Different feedback sensors, such as thermometers, are attached as a daughter board. The second daughter board controls the device, such as a reflow oven. It even includes PID auto-tuning for a more stable feedback loop.