PNEUMONOULTRAMICROSCOPICSILICOVOLCANOCONIOSIS (45 letters; a lung disease caused by breathing in certain particles) is the longest word in any English-language dictionary. (It is also spelled -koniosis.)

Hawkes Bay Tourism's Internet site says that Porangahau in New Zealand's South Island, "boasts the longest place name in the world: Tetaumatawhakatangihangakoauaotamateaurehaeaturipuk-apihimaungahoronukupokaiwhenuaakitan arahu, officially entered in the Guinness Book of Records."

This is the kind of thing you find out when you want to test how the margins of your blog are reset, using very long words.