HIDden is a Mac OS X Application and Preference Pane which can 'listen' to one or more Human Interface Devices (HID) and can take action based on what it 'hears.' You specify what action (open an URL, launch an application, type some text, etc.) to take for a paticular control (keyboard key, mouse button, etc.)
HIDden cannot (yet) remap the modifier (option, control, command) keys to be sent into the regular event chain, though it is something we would like to do. And if you want to remap your whole keyboard (e.g. switch the top row from QWERTY to ABCDEF), response time could be slow and you will have to set a lot of preferences.
In addition to keyboards and mice, other HIDs like joysticks should also be detected by HIDden. As mentioned in the history, there may be many more controls specified on a particular device than a user can activate. To help identify which ones are which, when the control panel is open, activating a control (e.g. pressing a button) will cause the corresponding item to be selected in the list.