Maybe not as influential as he was ground-breaking, I think Herschell Gordon Lewis 'the godfather of gore' deserves some mention. From what I understand he made the first totally bloody and gory films and some other interesting exploitation flicks. My personal fave by him is 'she devils on wheels'. (my band even covered the theme song!)
Here's a link with info on him:
http://www.phillyburbs.com/halloween2001/hgl/