The American Red Cross will accept donations of airline miles from American West, Continental, Delta, Northwest, or United.
Personally, I think this is the greatest thing since sliced bread. What a great way to actually do something constructive with the airline miles you have that you'll never use, or that will expire before you have squirreled away enough for even an advance-notice round-trip to an unpopular destination. So sorry, Boise, Idaho--my miles are going to those bitchslapped by Hurricane Katrina.
The procedure for each airline is different. Find out what to do by clicking here.