From here: Askoxford.com
We do have genuine (if rather obviously deliberate) examples in our files of antidisestablishmentarianism (28 letters) and floccinaucinihilipilification (29 letters), which are listed in some of our larger dictionaries. Other words (mainly technical ones) recorded in the complete Oxford English Dictionary include:

otorhinolaryngological (22 letters),
immunoelectrophoretically (25 letters),
psychophysicotherapeutics (25 letters),
thyroparathyroidectomized (25 letters),
pneumoencephalographically (26 letters),
radioimmunoelectrophoresis (26 letters),
psychoneuroendocrinological (27 letters)
hepaticocholangiogastrostomy (28 letters),
spectrophotofluorometrically (28 letters),
pseudopseudohypoparathyroidism (30 letters).
Most of the words which are given as 'the longest word' are merely inventions, and when they occur it is almost always as examples of long words, rather than as genuine examples of use. For example, the medieval Latin word honorificabilitudinitas (honourableness) was listed by some old dictionaries in the English form honorificabilitudinity (22 letters), but it has never really been in use. The longest word currently listed in Oxford dictionaries is rather of this kind: it is the supposed lung-disease pneumonoultramicroscopicsilicovolcanoconiosis (45 letters).

In Voltaire's Candide, Pangloss is supposed to have given lectures on metaphysico-theologo-cosmonigology (34 letters). In Thomas Love Peacock's satirical novel Headlong Hall (1816) there appear two high-flown nonce words (one-off coinages) which describe the human body by stringing together adjectives describing its various tissues. The first is based on Greek words, and the second on the Latin equivalents; they are osteosarchaematosplanchnochondroneuromuelous (44 letters) and osseocarnisanguineoviscericartilaginonervomedullary (51 letters), which translate roughly as 'of bone, flesh, blood, organs, gristle, nerve, and marrow'.

Some editions of the Guinness Book of Records mention praetertranssubstantiationalistically (37 letters), used in Mark McShane's Untimely Ripped (1963), and aequeosalinocalcalinoceraceoaluminosocupreovitriolic (52 letters), attributed to Dr Edward Strother (1675-1737).

This kind of verbal game originates, so far as records attest, with the ancient Greek comic playwright Aristophanes, inventor of Cloud-Cuckoo-Land (Nephelokokkygia).

The formal names of chemical compounds are almost unlimited in length (for example, aminoheptafluorocyclotetraphosphonitrile, 40 letters), but longer ones tend to be sprinkled with numerals, Roman and Greek letters, and other arcane symbols. Dictionary writers tend to regard such names as 'verbal formulae', rather than as English words.
This site is great for language trivia.