(Not Flyf15's post).
But courier travel is legitimate, even though it's relatively unknown. (that's a good thing).
I have a close friend that's registered with Now Voyager (located in NYC). There are also a few others such as Jupiter and WorldCour (not sure if that one is still around though). It goes something like this: You must already have a passport, you then register with the company for an annual fee (like, maybe $100/yr. or so). In addition to that fee, you pay a relatively small fee for the actual flight. You then call the company to tell them where you'd like to go and when (usually at a last-minute basis). They usually have special fights already arranged. All you have to do is tell them you're interested.
Almost all courier flights take you to intl destinations primarily. My friend used Now Voyager to go from DTW to NRT. They booked him on NW. He paid something like $500rt for last minute travel. He told me that the item he was accompanying was a document, of which he never saw or held. (It goes into the cargo hold even before you arrive at the gate). When he arrived at NRT he simply met a representative at the gate and signed a form.
From my understanding, the purpose for businesses using couriers is that many of the items are required to be accompanied by someone on the flight. It's much less expensive for that company to avoid using its own employees just for this purpose. That's where the courier travel service comes in. They act as a middle-man between the company and the traveling public, of which the company needs for the transport of their important documents or other small cargo.
Call up some of the courier companies and see for yourself. The Village Voice has listings of the companies every week in its travel section.