your wording is ambiguous. "Transponder" to me sounds like a remote control, which would explain the links in the first reply. What you're looking for is an RFID system.
Check out phidgetsusa.com, they sell USB RFID readers and all sorts of tags, including keychain/card/button/pill shaped, etc - with the USB reader you'd need to hook it up to a computer and let the computer handle the access control. You can also get RFID readers with an RS232 serial interface which would be easy to interface with a microcontroller if you want it to be a small standalone unit.