JAB Code
From Just Solve the File Format Problem
JAB Code (Just Another Bar Code) is a high-capacity 2D color bar code developed by Fraunhofer SIT that uses either 4 or 8 colors. The source code is available under LGPL v2.1
While current encoder/decoder is limited to 8 colors, the specification can handle up to 64 colors.
It does not have to be rectangular, able to take practically any shape.
The digital signature can verify authenticity of a code while offline.