Hitchhikers are letterboxes that travel from box to box with no permanent home of their own. They're also known as parasites in some circles, though in the United States the term hitchhiker is the standard. A hitchhiker includes the same basic components as a letterbox: a rubber stamp and a logbook either in a very small container or bag. It is found inside a host letterbox.

Stamp your personal signature stamp and the stamp of the host letterbox in the hitchhiker's logbook. Stamp the hitchhiker in your logbook and the host letterbox's logbook. The finder typically takes the hitchhiker and plants it in another letterbox.

Hitchhiker Etiquette Tips

• When planting a hitchhiker, make the stamp, container, and logbook small so it will fit well in a letterbox.
• When moving a hitchhiker to another letterbox, don't cram it in the letterbox. Wait until you find a container big enough for it to fit.
• When logging in to a small hitchhiker logbook, try to minimize the number of pages used. Consider stamping in only a portion of the stamps and try to squeeze images on as few pages as possible.
• If you know you won't have an opportunity to replant the hitchhiker in another location, don't take it. Leave it for the next finder to move it along. The goal is to move the hitchhiker along, not to have it sit in someone's bag at home.

[Source: Silent Doug's articles on hitchhikers that had been at letterboxing.info.]

Added 10/2017: Some people who plant hitchhikers intend for them to be a pleasant surprise for the finder. Hence, it is considered bad form to disclose where you have dropped off a hitchhiker -- unless it's your own, which you can announce to the world if you so choose. It's also considered bad form to leave someone else's hitchhiker in a hitchhiker hostel (HHH) unless the owner stipulates that doing so is acceptable; there's no surprise to finding a hitchhiker in a hitchhiker hostel.

Added 12/2007: Hitchhikers have now started to travel in postals. As with all HHs, if you question if it is only to travel in one type of box, contact the owner prior to changing box types. Many postal HHs stay with the ring they start in (And sometimes do not have their own logbook), while others jump from postal box to postal box (These have separate logbooks). They may also jump from postals into traditional letterboxes if the owner allows this. A good rule of thumb, "When in doubt, ask the box owner."

If you're considering launching a hitchhiker, you may want to consider making it a flea instead. A flea is much like a hitchhiker except there are more possible ways to move it, including just slipping it into some other letterboxer's pocket while he's not looking.