If all that you require is to be able to identify the user then what I do
is get the user to register with theie email address, generate a password for
them and store this information, which can be encrypted, in a database. Then I
email the user with their password. When they login I just verify the password
supplied with the one from the database.

If you want the user to be able to log in and out, and you want to be able to
recognise this on every page then cookies would seem to be the answer.