TL;DR
Mehrdad Majzoobi has developed an open-hardware near-field communication (NFC) add-on for Raspberry Pi that can function even when the board is powered off. This add-on, part of his Ubo Pod project, utilizes the NXP NT3H2211W0FT1 NFC chip, which offers both active and passive operation modes. In passive mode, it can harvest energy from a reader to communicate, making it useful for applications like tap-based payments without needing the Raspberry Pi to be on.