We were featuring a smallest DIY RFID tags on AVR and PIC. There are simple to implement when you understand microcontroller port structure. All you need is a coil wire to catch power and work as antenna. NYC Resistor shared a new build of smallest AVR RFID tag which replaces HID Prox cards.
It is built on Attiny85 microcontroller where coil is attached to PORTB1 and PORTB2 pins. The goal of this project were to make tag automatically serve as one of many tags, but it turned out that HID Prox readers use protocol that make it impossible to cycle through IDs. Using a select button for different card didn’t seem and option. This would require additional user action – the idea of automatics would be gone. So the only way out is to re-expose tad to magnetic field in order to try with different ID. Anyway project is great resource on building small tags.