I am designing a hobby project where I need a small device (powered by RPi that is running an app over linux) to be able to detect when it is within, say, 2 feet of another object. I think this is a perfect use case for RFID.

I need an RFID interrogator integrated with the RPi (I have a Model B), and tags that the interrogator is compatible with. When the reader detects a tag, it needs to communicate with an app running on the RPi, probably via serial comm.

My question: What types of RFID tags and readers would be cheap/COTS, and compatible with RPi?