Md11dc10 is actually completely correct;
FedEx I know does a thing where you can fly in the jumpseat providing you meet certian requirements (IE no criminal record) on any flight that FedEx operates;
There are also, on 747F aircraft, several seats in the upper deck, and airlines such as Air France Cargo sometimes offer seats at rock bottom prices to passengers who wish to fly.
I have a couple of friends who say that FedEx is the only way to travel! And the only downside is that sometimes they fly at ungodly hours of the day. But beyond that, its fantastic.
I am going to try to get on a Freighter from London Stansted to Atlanta in a couple months time, or, failing that, from LHR to JFK, to avoid the one-way price penalty of the passenger airlines. If I succeed I will let you know how it goes!