These teeny-tiny little NFC LED stickers power themselves when in an electromagnetic field, and they'll add a whole layer of interactivity to your projects.
NFC (Near Field Communication) is a way for two devices very close to each other to communicate. It's like a very short range Bluetooth that doesn't require authentication. NFC is a special subset of RFID.
You can light these up with NFC on your (Android) phone, card readers at the shops, or for something more homebrew then try the Adafruit NFC/RFID shield!
Note: these are not NFC data tags (if you want tags, then we have them too), they just light up when in the vicinity of a NFC signal.