A few years ago I did some work on a project which used RFID (Radio Frequency IDentification) to do all sorts of cool things. There were several problems with this, though. The first was that even high-quality UHF RFID readers and antennas would sometimes struggle to see and recognize the "tags" we had attached to various objects as we ran through doorways and the like. The second was the cost of getting involved. Readers and antennas cost hundreds or (more often) thousands of pounds, and despite having their own intelligence of sorts, these readers also required a "proper" computer to control them.
At this point RFID was a field that only organisations with relatively deep pockets could afford to play in.
Now things seem quite different. The Raspberry Pi has significant processing power for RFID reader control, and readers ("near field", 125KHz, at least) can be had for less than £10 on ebay and can easily be interfaced either using USB or by direct serial connection (see here and here). Adafruit sells relatively low-cost RFID hardware, and offers a tutorial on how to use them.
All of this makes me wonder what I might be able to do with a few cheap NFC readers and a bunch of tags.