The AT90USBKey is a demo board produced by Atmel to show off their wonderful at90usb1287 microcontroller. It is also an inexpensive base to build a HID device.
There are two tri-coloured LEDs on the board. Their default use is to show the board status. They can be used as output values of a HID device.
A small switched joystick/directional switch/hatswitch with built-in button is included on the board. It can be used as a directional switch and button of a HID devcice.
A Negative Temperature Coefficient (NTC) thermistor, R29, provides an analog temperature reading. A thermistor is a resistor whose resistance changes with temperature. An NTC thermistor's resistance will decrease when the temperature increases. This can appear as an input value on a HID device.
Data Flash Memory
The external flash memory allows the USB Key to be used as a flash drive. This is one of the demo applications that can be loaded on to the USB Key. Data flash is not supported as a HID resource.
HWB - Hardware Boot Loader Button
The HWB button is connected to pin PE2 on the USB Key and is used to force the microcontroller into boot loader mode at start up. If this button is depressed when the microcontroller comes out of reset, either when the reset button is pressed, or the device is powered up, the microcontroller will run the boot loader firmware. Because the HWB functionality is only used at start up, it can then be used for another purpose. However, you must be aware that there is an external pull-up resistor and capacitor connected to the shared pin, restricting it's use to switch devices that can use the pull-up resistor.