A pre-programmed 'duino microcontroller loaded with firmware to make it operate as a KISS TNC. This isn't a simple Arduino project, but a re-flash of the firmware allowing you to use it for AX25 Packet and APRS. Loading the new firmware requires use of a 3rd-party command-line tool, AVRDude (or XLoader) and may not be for the novice, so we've done the hard work for you. Just add a few resistors, capacitors and an NPN transistor for the PTT (or use VOX).
Difficulty : 4/5 - Some additional components are required to complete this project so you'll require a little background knowledge of APRS, soldering and finishing the circuit.
Once you've installed the CH341 driver (link above) and configured AGW Packet Engine (or any KISS-enabled software), you can start to enjoy AX25 or APRS with this cheap device. We use ours with AGW Tracker and a Baofeng BF-F8 via a battery eliminator so it runs off 12v 24/7.