RFID reader technology and cloning tags!

RFID tags are available in a few different standards ranging from the very secure ones with data and encryption, to the much simpler ones that simply present themselves as a unique (or not so unique) serial number to the RFID reader.
This is a look at the cheapest and most common, that is widely available on ebay from a wide range of suppliers as either bare components or packages for full door control. Not that I'd trust these much for security with people like us about...