I have a metal dog tag (one actually for dogs) that I wear around my neck whenever I bike. It has my name, my address, my wife's name and phone number, the drugs I take, that I have no known medical allergies, and my medical insurance number.
I don't see any reason to have a SSN or Passport No. on the tag as medical personal will not be able to access that information easily. While a link to a webpage is a good idea, I'm not sure it will help in an emergency. Also, I doubt that doctors would trust someone's dog tag/Road ID for blood type as they have quick tests for such things.
The tag I have I ordered over the web for a few dollars and it came with a chain, though the amount of information I can put on it is limited.