A little board that transparently relays CAN messages from one bus to the other. A wire, so to speak. That seems useless.

But wait, there's more. You can modify the CAN messages in-flight. This is effectively a man-in-the-middle attack on CAN bus. The board has three CAN buses, so if you want, you can actually do two CAN bus MITM attacks at once, or use the third CAN bus as a private bus for debugging, filtering, etc.

The board natively supports logging CAN messages over its USB port using LUFA's USB-CDC implementation (you might need drivers on some OSes).

There is no API or anything like that; if you want to modify the board's behavior, you need to program this in yourself. This is done in Atmel Studio 7 using the sample project file supplied under the source code link. You will also need a programmer, e.g. an AVRISP mkII or any other programmer capable of PDI programming. The sample project already implements everything you need to get started and has some example lines showing how to modify CAN messages and receive commands and/or send debugging information over USB.

The hardware is open source, available as TinyCAD schematic and FreePCB layout in the source code link. There are some additional features in hardware that you may want to use, like an expansion header with analog/digital I/O.

This board is a fully assembled product.