The Arduino Uno (http://arduino.cc/) is an open source hardware micro-
controller designed primarily for prototyping and experimentation.
Although the devel/arduino port already exists for programming the device,
it will not work properly with the newest Arduino hardware.  Previous
versions of the Arduino used an FTDI USB to Serial interface.  The newest
Arduino (beginning with the Uno) uses an on-board ATMel 8U2 controller
to emulate a USB to Serial interface with its own custom Vendor ID and
Hardware ID.  As a result, NONE of the existing FreeBSD USB to serial
drivers can work with it.  This kernel driver supplies the necessary
kernel support for the Arduino Uno on FreeBSD.

Additionally, some 'ACM' USB Serial devices may work with this driver by
manually adding their Vendor ID and Product ID combination to files/ids.txt

Official web site