The Combat Application Tourniquet, or CAT Tourniquet, is a modern military tourniquet used by several countries and emergency services.
The CAT is intended to be used in extreme circumstances to treat severed limbs or stem very heavy bleeding. It can be used with one hand (say if you lose your arm).
The CAT is extremely durable and simple to use, with built in windlass rod to tighten the tourniquet, and plastic guards to hold the windlass in place.
You need to check around to find a good price, as the CAT is usually sold in bulk (it's issued to every US and Canadian soldier that serves overseas), but its strongly recommended for keeping in an advanced first aid/trauma bag (something every motorist should have in their car.
Be warned, do your research on tourniquet use, because if improperly used (owie papercut), and if the wound isn't treated by higher medical authorites, the limb can die within several hours (if you have to keep it on that long, loosen the tourniquet and recheck the injury every hour.