Note this section is under major revision and expansion, it may be merged with demo boards or not.
Modules ( at least as we use the term ) are circuit subsystems that typically consist of many components that perform some function and are plugged into a larger circuit where they perform a service or act as a subsystem in a larger system. Often they are an efficient way of incorporatiing some functionality without the bother of discrete components. In the past, and still, this category can blend in with demo boards where manufactures put some of their components on a board to make it easier to evaluate and/or the component. The Arduino is much like the old demo boards for microcontrollers, but was built with the idea that it would be used as a module in a larger project. This idea is extended in the arduion enviroment by the idea of shields which are modules that plug directly into the Arduino. In some sense the module is an extension of the IC ( or vise versa ).
We will group these modules by function.
Microcontroller and Computer
We talk about RC servos themselves at Motors#Servo_Motors.
We talk about various more-or-less standardized signal protocols and the electronics to deal with them at servo control.
See Motor driver
Other demo boards