The Other Others as it seems some call them (http://lostpedia.wikia.com/wiki/The_other_Others) have never been shown to us who they are and most likely never will but I have a theory that we have already been shown (in a sense) who they were. Sayid, Jack and Locke.
I was re-watching "S06E14 - The Candidate" and this came to my mind. When Jack wakes up on the outrigger to see Sayid, Sayid is sitting there cleaning his gun (As he would if it was recently used). This outrigger went to hydra Island With 3 Passengers, Jack, Sayid and Locke, and When the Losties were flashing through time it was some time in 2007 or later when they were attacked by the Mysterious Other Others.
So in My opinion that is who I feel the Other Others are, and if we don't get an official answer, I think this may fit.