MDK-Middleware provides a USB Device Reference Example that contains following projects implementing specific USB Device functionality:

USB Device HID example uses USB HID class to control LEDs and push buttons on the development board.
USB Device Mass Storage example implements a mass storage USB device using internal RAM as storage media.
USB Device Virtual COM Port example provides a virtual communication port to UART bridge to the USB Host.

See CMSIS-Toolbox - Reference Applications to learn more about the reference application concept in Open CMSIS Pack used by these examples.