It is over, your search is done! Let fate choose now, the righteous one.
"Over here, you idiot!" ring a bell? We've had a bunch of readers ask us lately just who does the voice for the Headless Horseman -- corny poetry aside, players are really loving the laugh and timbre of the guy who doesn't even have a throat to speak with. The most common guess we've heard is Gerard Butler, star of 300, but that doesn't really compute, as Blizzard usually uses lesser known voice actors and even most of their sound crew and artists (Chris Metzen does quite a few voices, actually, including most of the Orcs in game).
We've got two guesses for the Headless Horseman: the first is voice actor Michael McConnohie -- he's done a few other voices for Blizzard before (including Kel'thuzad). While his voice is definitely recognizable (he sounds like the Human male to me), the other strong male voice we know has worked for Blizzard is Mark Graue, who runs a voice studio in LA, and has worked on WoW since the beginning (he did the Undead emotes). Here's Graue's demos -- he sounded to me like the Horseman for sure when I first listened, but a few times in, I'm not so sure any more.
You can listen to the voice demos for both those guys and judge for yourself. Again, those are just guesses, based on who's worked with Blizzard in the past -- we don't really know for sure who it is. But we do have a query in to Blizzard on just whose manical laugh that is you hear when you throw a Jack-o-Lantern on your friend's head, so if they get back to us with a name, we'll post it here.