Letterboxing North America - Includes a FAQ, links, glossary, and clues for letterboxes all over the United States. - http://www.letterboxing.org/
Silent Doug's Letterboxing Site - Articles including getting started, outdoor ethics, a glossary, and an explanation of cryptic letterboxer codes. Also includes a list of boxes found and clues for boxes placed. Most clues are for boxes in the northeastern region of the United States. - http://www.letterboxing.info
Have Feet Will Travel - An introduction and guidelines to finding letterboxes on Dartmoor. Includes details of National Park by-laws and contact details for checking military activity. - http://easyweb.easynet.co.uk/~have.feet/boxing/boxing.html
Letterboxing Kids - Includes a map-reading game, photo album, story and instructions for making rubber stamps. - http://www.letterboxing.org/kids/