This article is from the Problem Behaviors in Cats FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Put something distasteful on the cord to discourage chewing. Substances to try: tabasco sauce, cayenne pepper, Bitter Apple/Orange, nail-biting nailpolish, orange/lemon peel. If you cannot find a substance that will repel your cat, you may wish to use gaffer's tape to secure exposed cords. Gaffer's tape comes in a variety of colors and you will probably be able to find something relatively inconspicuous. (Duct tape can also be used, but it's pretty messy and hard to clean up if you're moving out of an apartment. Consider permanent wiring if you own your place: installment behind walls, etc.
This will work on other chewed items, as well, although you will need to check the compatibility of the chewed item with the substance you put on it.