Bit bodge-tastic but if you're in a rush and don't have the requisite tool or (as I used to be) too skint to buy too many tools that only get used occasionally:
Pair of pinchers (pincers? nail pliers?) - hold them on a solid surface (vice) and whack the jaws with a hammer - makes a lovely clean cut.
Of course not practical if you're cutting the loose end off a cable already fitted in the car











