It's unbelieveble how few people know about grammatic.
In the clause "It is I", "It" is the subject, "is" is the copulative verb and "I" is the predicative, which must be in nominative case, given the fact that English stil keeps cases for personal pronouns, such as I, He, She (me, him, her).

So, grammatically "It is I" is the only correct clause; neverthless, in the informal spoken English people usually say "It's me".

P(ost) S(criptum): in the clauses

It is I who knocked on the door

It is me you are looking for

you should notice that there are two cluases in the same period:
It is I (or me)
who you are looking for

so that "I" is still the predicative (in nominative case) for the first clause and is the complement depending on "looking for"

after the verb "to be" you always have a nominative, even in the second one, because in the second one you avoid the relative pronoun

It is I "who" you are looking for
or

It is I for whom you are looking