Open source PCB the size of a credit card that is capable of detecting the field generated by a RFID reader and identify if it is a LF(125kHz) or HF/NFC(13.56MHz) reader.
https://github.com/exploitagency/RFID-Field-Detector

This is a tiny little sticker designed to be stuck to a fingernail prior to covering it with nail art.
It then flashes with pulses of power received from the NFC (Near Field Communication) coil in the back of your phone. I’m not sure if they are intended to be used with an app that makes the NFC coil more active, but I wouldn’t really want a random app like that on my phone anyway, for security reasons.

Note that subsequent closer examination showed that it has a second 12 turn coil on the back to effectively double the number of turns to 24.

These stickers can also be used to locate the position of the coil in your phone by shifting them about and noting the intensity. They may also be effective on the coils of NFC readers, but do not work with the classic RFID 250kHz readers or QI charger coils.

The circuitry is interesting in the way it uses a simple voltage doubler based on high speed diodes and capacitors.

took some of the RFID tags back to my apartment in Shenzhen, and started taking things apart!

When I took apart the RFID tag from the cashierless store I ran across in Yiwu, China, my friend Stuart challenged me to try and make my own RFID tag. And it sent me on adventure that took me around the world, to meet some really amazing people!

Dave does an impromptu teardown and repairs his 125KHz RFID lab access card.
And finds a use for his DSO Quad oscilloscope.

[Sjaak] is back at it again with the cool PCB business cards, this time alleviating the burden to physically type his contact information into your phone. But NFC isn’t the only cool thing on this PCB – as always, his aesthetics don’t disappoint.

PCB Businesscard Nextgen: NFC enabled
http://smdprutser.nl/blog/pcb-businesscard-nextgen-nfc-enabled/

Near Field Communication (NFC) is something we take for granted these days. Nearly all smartphones have it. We even have NFC interfaces for all our favorite development boards. NFC’s history goes back all the way to 1997, when an early version was used in Star Wars special edition toys. Radio Frequency Identification (RFID), which NFC builds on, goes back even further. The patent citation trail leads all the way back to 1983 in a patent awarded to [Charles Walton]. NFC is much more than RFID though. The idea of two way communication between devices opens up tons of possibilities for projects and hacks. This week on the Hacklet we’re checking out some of the best NFC projects on Hackaday.io!