Hammer Strength makes a decent one....unlike the old-school hack squat machines (which were basically upright sleds) the newer ones allow for the hips to travel toward the rear, in a somewhat more natural position (as opposed to simple up and down linear travel).
I doubt anyone would recommend any squat machine over a free-weight version, but it's good every now and then for variety or when the rack is being dominated at 6 pm.
If you mean an "at home" version then I'm not sure.
Anyone ever use one of those hip-belt type squat devices? I always wanted to try one of those.