You know how some of the characters have official smilies, like these?
Well, I've seen unofficial ones floating around too, like these:
But I couldn't find any list of them, and I doubt my collection here is comprehensive. (In fact, all but one of these I found on a single unrelated thread.) While I don't think I'm up to keeping such an index updated myself, for what it's worth everyone should probably post whatever ones they have or know of. Then, if anyone should take up the task of tending a proper index, I think it should probably be linked to in the stickied "threads of notice".
(Hm... Maybe we should list all characters without smiley heads too, so that we know who still needs one?)
Fawkes maintains a collection of smilies over in the Order of the Stick forum. That probably fills the niche of an index, and it's got a list (albeit a slightly outdated one) of wanted smilies in the second post too.