After the Minneapolis bridge collapse, my wife asked me if we could get some seat belt cutters "just in case". She wanted some extra confidence, in case she was ever trapped in her car, or needed to help someone else out.
I did a Google search, and found a very nice tool made by Gerber, which also makes the excellent Multi Plier multi-tool (second only to the high end Leatherman tools). Amazon sells it (here) for a very reasonable $5 plus shipping.
The knife has a simple and easy to use design, with a cutting hook on one end, made from two opposed straight blades, a body made of tough black plastic, and a nylon lanyard at the other end. To use it, you hook the cutting end over a seatbelt and pull, drawing the two blades through the fabric.
Once you get one of these, another important consideration is where to keep it. If it is in your trunk, or the back of your glovebox, it won't do you much good. A better way to keep it is to use duct tape or velcro tape to affix the cutter to the underside of the driver's seat, or a similar out of the way surface, where you can grab it quickly and easily.
Will you ever need a seat belt cutter? Probably and hopefully not. But for $5, it is a cheap bit of extra insurance.
P.S. I also keep a center punch, to easily break safety glass with. $10 at any hardware store.