Hunters are incredibly dangerous foes, deployed more like equipment than soldiers. They are brought in by the Covenant for demolition or heavy defense, and always work in pairs. These massive creatures appear to be composed of multiple organisms that exist within the Hunter armor, creating a hive creature in bipedal form. Near-impenetrable armor and a devestating hand-held plasma weapon make Hunter pairs very problematic.
NOTE: Not much new here about Hunters. Gameplay-wise, these guys underwent a major overhaul between Halo 1 and Halo 2 that removed their extreme vulnerability to fire against the sensitive chinks in their armor. It seems reasonable to assume that graphically Bungie is pretty happy with their design at this point and that there won't be a major facelift-- nothing as radical, say, as the change in Jackals from Halo 1 to Halo 2.