The longest word in the dictionary containing no letter more than once is "smiley." (There is a mile between the first and last letter.)

But if you just want long words, the usual response is antidisestablishmentarianism, and this can be found in most dictionaries.

The longest town in the United Kingdom is in Wales. It is:

Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch.

This name is given in full on the railway station. The local people do not bother to say the name in full. They refer to it as Lannfair PG.

However, there is a place with an even longer name in New Zealand. It is the name of a hill. Here it is:
Taumatawhakatangihangakoauauotamateaturipukakapikimaungahoronukupokaiw en uaitnatahu.

Longest disease is pneumonoultramicroscopicsilicovolcanokoniosis, defined by the OED as "a factitious word alleged to mean 'a lung disease caused by the inhalation of very fine silica dust' but occurring chiefly as an instance of a very long word.

However, the longest official word ever (1,913 letters) is the term for the formula C1289H2051N343O375S8
And it is spelled:
Methionylglutaminylarginyltyrosylglutamylserylleucylphenylalanylalanylglutaminylleucyllysylgl
utamylarginyllysyglutamylgycylalanylphenylalanylvalylprolylphenylalanylvalylthreonylleucylgl
ycylaspartylprolylglycyllisoleucylglutamylglutaminylserylleucyllysylisoleucylaspartylthreonylleu
cylisoleucylglutamylalanylglycylalanylaspartylalanylleucylglutamylleucylglycylisoleucylprolylph
enylalanylserylaspartylprolylleucylalanylaspartylglycylprolylthreonylisoleucylglutaminylaspara
ginylalanylthreonylleucylarginylalanylphenylalanylalanylalanylglycylvalylthreonylprolylalanylgl
utaminylcysteinylphenylalanylglutamylmethionylleucylalanylleucylisoleucylarginylglutaminyllys
ylhistidylprolylthreonylisoleucylprolylisoleucylglycylleucylleucylmethionyltyrosylalanlylasparagi
nylleucylvalylphenylalanylasparaginyllysylglycylisoleucylaspartylglutamylphenylalanyltyrosylal
anylglutaminylcysteinylglutamyllysylvalylglycylvalylaspartylsrylvalylleucylvalylalanylaspartylv
alylprolylvalylglutaminylglutamylserylalanylprolylphenylalanylarginylglutaminylalanylalanylleu
cylarginylhistidylasparaginylvalylalanylprolylisoleucylphenylalanylisoleucylcysteinylprolylprolyl
aspartylalanylaspartylaspartylaspartylleucylleucylarginylglutaminylisoleucylalanylseryltyrosylg
lycylarginylglycyltyrosylthreonyltyrosylleucylleucylserylarginylalanylglycylvalylthreonylglycylal
anylglutamylasparaginylarginylalanylalanylleucylleucyllysylglutamyltyrosylasparaginylalanylal
anylprolylprolylleucylglutaminylglycylphenylalanylglysylisoleucylserylalanylprolylaspartylgluta
minylvalyllysylalanylalanylisoleucylaspartylalanylglycylalanylalanylglycylalanylisoleucylserylgl
ycylserylalanylisoleucylvalyllysylisoleucylisoleucylglutamylglutaminylhistidylasparaginylisoleuc
ylglutamylprolylglutamyllysylmethionylleucylalanylalanylleucyllysylvalylphenylalanylvalylgluta
minylprolylmethionyllysylalanylalanylthreonylarginylserine

a longer word in the dictionary that has no repeated leter is courage OMG god @_@ SD_Frogfeet2@cox.net

that word is incorrect, you only have 1,859 letters!!

The longest word in the English dictionary, if you get a good one, is:

Pneumonoultramicroscopicslilcovolcanokoniosis

Due to the way the english language works, there is no longest word in the dictionary. The way that subject verb clauses and everything else fit together, leaves open the possibility for endless words. For verification, see Steven Pinker's, "The Language Instinct".