Firstly, there are technical words in science and medicine that are just strings of jargon elements. The longest word in The Oxford English Dictionary (TOED) is such a word. It is pneumonoultramicroscopicsilicovolcanoconiosis, (45 letters) as listed by SDDS above. The longest of these technical monsters apparently opens with the letters �methionyl...' and goes on for a further 1900 letters!
Secondly, there are joke words that have been made up at various times throughout history, specifically striving for great length. The oldest of these - created at Eton in the 18th century - is probably floccinaucinihilipilification, (29 letters) listed in TOED, which means setting things at little value. A more modern example of such a joke word is supercalifragilisticexpialodocious (34 letters) - also listed in TOED - from the film, 'Mary Poppins'.
Thirdly, there are genuinely long words which have emerged to fill a particular gap in meaning. The longest of those is antidisestablishmentarianism, (28 letters). This might be considered the longest �normal' word in English, in the sense that it is - of the four so far listed - the only one you are at all likely actually to see in print.
So, if you are looking for non-jargon/non-jocular words that you may well see in print one day, the longest is certainly 'antidisestablishmentarianism'. (Because of Prince Charles's decision to marry Mrs Parker Bowles and given the fact that he will one day be head of the "established" church in England, the very word appeared more than once in the British press in early 2005.)
However, there is no reason why there shouldn't exist, perhaps, an adverb such as 'antidisestablishmentarianistically' to describe how believers in antidisestablishmentarianism might behave.