The Asphyx (1973) is a film that has a fantastic storyline, but lacked budget and tech to really make it what it could have been. I HATE remakes, but I'd actually love to see this film remade properly with a good budget and A List F/X team. If you have not seen it, you might want to check it out.
The story involves capturing the spirit as it leaves the body when death occurs. Scientists manage to successfully capture the soul, but the results are less that pleasant.