RFID button is greatly used in sensing presence, identification, and these kinds of aspects. Some great characteristics include the following:
• Thumbnail-size RFID tag (can even be sewn into clothing or put into enclosures)
• Comes with a unique 32-bit ID code and is not reprogrammable
• Carrier frequency of this tag is 125 kHz, which makes it perfectly working with ID-2, ID-12 and ID-20 RFID readers.
It has also been tested with one of the ID-12 readers and measured a maximum read distance of about 32mm, which is fascinating really.
[Product pictures and descriptions are courtesy of Sparkfun.com]