I personally have a spring loaded rescue tool with seatbelt cutter mounted on my door (lifehammer evolution), one old fashion hammer with cutter on the center console (lifehammer classic, as delivered by the car dealership) and a reqme on my car keys.
I just got the Life Hammer Evolution, and I really like how easy it is to mount on your car door. It looks great on my car! It's going to replace the previous edition of Life Hammer -- mounting that requires drilling holes in your car, and I ended up just using duct tape. Thanks for alerting me to it! I'm slightly concerned about how securely the mount will hold on to the hammer if the car flips over. Any thoughts on this?
I think ireckon is right: if you have a good EDC knife, it should also be able to handle seat belts. I personally prefer a partially serrated knife for cutting rope and webbing.