To come back with a predictable response: everyone says "who" even if they mean "whom". Using "whom", even if it is grammatically correct, is perceived to be wrong in most conversational situations, or other informal situations like Twitter.

Anyway, the quick guide: if you would say "him/her", use "whom", and if you would say "he/she", use "who".

"Who is that?" ... "He is the programmer."

"To whom is the message addressed?" ... "It is addressed to him."

He -> Who / Him -> Whom

But who wouldn't say "Who is it addressed to?" You'd sound like the posh bloke from an Ealing Comedy if you used "whom" all the time.