MiFare Classic (13.56MHz RFID/NFC) Clear Keychain Fob - 1KB

This is a blank MiFare Classic keychain fob - often used for electronic locks or customer identification but also found in other systems where a small and easy to carry tag is desired.
The tag contains a NXP S50 chip and an antenna, and is passively powered by the reader/writer when placed a couple inches away.
These can be read by almost any 13.56MHz RFID/NFC reader but make sure it can handle MiFare cards as there are a few other encoding standards (like FeLica) They are tested and work great with both our PN532 NFC/RFID breakout board and Adafruit NFC/RFID Shield for Arduino!

These chips can be written to
can store up to 1 KB of data in writable EEPROM divided into banks
can handle over 100,000 re-writes.
You can use our PN532 NFC/RFID breakout board or Adafruit NFC/RFID Shield for Arduino to read and write data to the EEPROM inside the tag.
There is also a permanent 4-byte ID burned into the chip that you can use to identify one tag from another

the ID number cannot be changed.

These use the S50 chipset, which used to be the 'classic' NFC chipset. In ~2014, the NFC forum decided not to support this chipset anymore, so newer phones do not support the MiFare classic. This only matters if you're trying to use this tag with a phone/tablet.
S50 chip specification:

1 KiloByte (8 KiloBit) non-volatile EEPROM storage
Built in encryption engine with 48-bit key
4 Byte unique identifier burned into the chip
13.56 MHz frequency

Tag specification:

30mm x 25mm x 6mm / 1.2" x 1" x 0.2"
Weight: 4.3g
Works about 2" away from reader

Downloads: NXP S50 datasheet