Prodos and Hasslefree also come to mind, though it would be better for both companies if they had a professional account for just their announcements and a private account for discussions.
Both Gangfight Games and (the now defunct?) On the Lamb Games run their official forums from here. Victoria Miniatures uses these forums as well, she always refers to Dakka Dakka as the place to discuss her products.
There was also a Games Workshop USA rep who created an account on these forums for the recruitment of store staff, he even became a
DCM, but sadly the more vitriolic elements of this forum have made him feel less than welcome.
All in all though, smaller companies more likely than not represent their selves, whereas bigger companies like Infinity and Privateer Press don't bother with us scrubs, but are instead run by volunteers.