RFID is a really great way but most off the cheap applications only work up to 15centimetres. That is ony 1/4 of what you need.

This Olimex, produced in Europe emulates a keyboard input, which the Pi can do over USB. Its max range is 15cm though but cheap [at 5 Pound Sterling][1] and works with tags on **125Khz**

![enter image description here][2]

[Here is a system that can read certain][3] RFID **UHF (~900Mhz)** tags up to 4 metres, that is way over what you need. There is no price so I assume its darn expensive and it doesnt support all tags and uses some kind of proprietary SDK, over USB and is supported on Linux. These are typically used in Toll Gate systems where cars have E-Tags


  [1]: http://www.ebay.co.uk/itm/Olimex-MOD-RFID125-usb-keyboard-emulating-125KHz-RFID-tag-reader-also-RS232-/201111006524?_trksid=p2054897.l4275
  [2]: https://i.sstatic.net/0lIm6.png
  [3]: http://www.tertiumtechnology.com/smartantenna.php