Who are the dead and/or gone?

Prior to seeing Locke roll out of the crate many of us had different theories on what the dead and gone people were.. for example we may have thought the Island itself was creating them.. the Smoke Monster was turning into them.. it was Locke and his crazy drugs making people see stuff.. Walt's got mad skillz' and he can appear wherever he wants to.. etc.. now it's the mystery man using peoples bodies i.e. Locke and Christian.. So my question is Who are the dead and/or gone? Let me poll you guys and see what we come up with... of course comments are welcome too. I am just interested in what most of us believe.