Serial and Ethernet are the easiest to get going but they are far from the only gadgets the Linux kernel supports. You can also try such options as:
- Mass storage (you can have the Pi appear as a 'USB key' disk drive ) - note, we didn't get this up and running smoothly, it enumerated but disk access to the backing file didnt work on our windows machine
- MIDI - shows up as a 'native' USB MIDI audio device
- HID - appear to the host computer as a mouse/keyboard/joystick
- Audio - Show up as an audio/speaker device & line in as well?
- Composite - a mix of serial/ethernet/mass storage composite devices is available. Note that this may work on a Mac or Linux but for windows you'd need a custom driver
- Printer, webcam, etc - There's about a dozen more options
For more details, check out the USB gadget API framework page
We compiled all of the available USB gadget modules into the December 25, 2015 (or later) kernel tgz. You can enable them by using modprobe or editing the /etc/modules file to enable. If they need options, creating a new file for those options in /etc/modprobe.d/usbgadget.conf or similar
In particular, here's the modules that are available:
# # USB Peripheral Controller # # CONFIG_USB_FUSB300 is not set # CONFIG_USB_FOTG210_UDC is not set # CONFIG_USB_GR_UDC is not set # CONFIG_USB_R8A66597 is not set # CONFIG_USB_PXA27X is not set # CONFIG_USB_MV_UDC is not set # CONFIG_USB_MV_U3D is not set # CONFIG_USB_M66592 is not set # CONFIG_USB_BDC_UDC is not set # CONFIG_USB_NET2272 is not set # CONFIG_USB_GADGET_XILINX is not set # CONFIG_USB_DUMMY_HCD is not set CONFIG_USB_LIBCOMPOSITE=m CONFIG_USB_F_ACM=m CONFIG_USB_F_SS_LB=m CONFIG_USB_U_SERIAL=m CONFIG_USB_U_ETHER=m CONFIG_USB_F_SERIAL=m CONFIG_USB_F_OBEX=m CONFIG_USB_F_NCM=m CONFIG_USB_F_ECM=m CONFIG_USB_F_EEM=m CONFIG_USB_F_SUBSET=m CONFIG_USB_F_RNDIS=m CONFIG_USB_F_MASS_STORAGE=m CONFIG_USB_F_FS=m CONFIG_USB_F_UAC1=m CONFIG_USB_F_UAC2=m CONFIG_USB_F_UVC=m CONFIG_USB_F_MIDI=m CONFIG_USB_F_HID=m CONFIG_USB_F_PRINTER=m CONFIG_USB_CONFIGFS=m CONFIG_USB_CONFIGFS_SERIAL=y CONFIG_USB_CONFIGFS_ACM=y CONFIG_USB_CONFIGFS_OBEX=y CONFIG_USB_CONFIGFS_NCM=y CONFIG_USB_CONFIGFS_ECM=y CONFIG_USB_CONFIGFS_ECM_SUBSET=y CONFIG_USB_CONFIGFS_RNDIS=y CONFIG_USB_CONFIGFS_EEM=y CONFIG_USB_CONFIGFS_MASS_STORAGE=y CONFIG_USB_CONFIGFS_F_LB_SS=y CONFIG_USB_CONFIGFS_F_FS=y CONFIG_USB_CONFIGFS_F_UAC1=y CONFIG_USB_CONFIGFS_F_UAC2=y CONFIG_USB_CONFIGFS_F_MIDI=y CONFIG_USB_CONFIGFS_F_HID=y CONFIG_USB_CONFIGFS_F_UVC=y CONFIG_USB_CONFIGFS_F_PRINTER=y CONFIG_USB_ZERO=m CONFIG_USB_AUDIO=m # CONFIG_GADGET_UAC1 is not set CONFIG_USB_ETH=m CONFIG_USB_ETH_RNDIS=y CONFIG_USB_ETH_EEM=y # CONFIG_USB_G_NCM is not set CONFIG_USB_GADGETFS=m CONFIG_USB_FUNCTIONFS=m CONFIG_USB_FUNCTIONFS_ETH=y CONFIG_USB_FUNCTIONFS_RNDIS=y CONFIG_USB_FUNCTIONFS_GENERIC=y CONFIG_USB_MASS_STORAGE=m CONFIG_USB_G_SERIAL=m CONFIG_USB_MIDI_GADGET=m CONFIG_USB_G_PRINTER=m CONFIG_USB_CDC_COMPOSITE=m CONFIG_USB_G_ACM_MS=m CONFIG_USB_G_MULTI=m CONFIG_USB_G_MULTI_RNDIS=y CONFIG_USB_G_MULTI_CDC=y CONFIG_USB_G_HID=m CONFIG_USB_G_DBGP=m # CONFIG_USB_G_DBGP_PRINTK is not set CONFIG_USB_G_DBGP_SERIAL=y CONFIG_USB_G_WEBCAM=m # CONFIG_USB_LED_TRIG is not set # CONFIG_UWB is not set CONFIG_MMC=y # CONFIG_MMC_DEBUG is not set
