What is the longest English word with all unique letters (i.e. no letter can appear more than once)?
“AMBIDEXTROUSLY is the longest word in common use with no letter appearing more than once. SUBDERMATOGLYPHIC is the longest word with no letter appearing more than once; it is not found in any dictionary, but does occur in a published medical article in Annals of Dermatology. Others are: MISCONJUGATEDLY, UNCOPYRIGHTABLE, which is found in printed publications from the Library of Congress Copyright Office, DERMATOGLYPHICS, SCHIZOTRYPANUM, and VESICULOGRAPHY, all three of which are found in Stedman’s electronic medical dictionary, and BRICKLEHAMPTON (a village in Britain near Evesham) [Chris McManus, Ted Clarke]. Other 14-letter words are UNDISCOVERABLY, which is in W3, BENZHYDROXAMIC, HYDROMAGNETICS, HYDROPNEUMATIC, PSEUDOMYTHICAL, SCHIZOTRYPANUM, SULPHOGERMANIC.”
From here: http://plex.us/archives/word.html
There’s several names for this phenomenon, but perhaps the best is heterogram.
The longest placename heterogram is MALITZSCHKENDORF, Germany (16 letters). Another long one is GUMPOLDSKIRCHEN, Austria (15). The longest in the English-speaking world is HACKSTOWN BRIDGE, Ireland (15). There’s about half a dozen 14-letter placename heterograms in the US, but all of them are two or more words (examples: South Cambridge, NY; Buckingham West, FL; West Rockingham, NC). The longest single-word one is BRIDGEHAMPTON, New York (13).
This page has more on them (down towards the bottom).
Just looked at the link Scruloose provided. It’s a rather old (like 8 years old) archived version of the same page I provided. Use the current page(s), please.
Here are the longest ones (14 letters or more) I found in the linux /usr/share/dict/words file (version 3.0-7)
ambidextrously, enzhydroxamic, dermatoglyphic, dermatoglyphics, hydromagnetics, hydropneumatic, pseudomythical, Schizotrypanum, subformatively, sulphogermanic, undiscoverably.
And here are a few hyphenated ones from the words file (these seem to be a bit more fun):
cloth-measuring, cloth-spreading, hydro-pneumatic, wheat-producing, breath-sucking, clothes-drying, crystal-winged, double-spacing, flower-sucking, heat-producing, hot-vulcanized, loud-screaming, pseudo-African, self-combating, slow-breathing, spring-touched, stocking-frame, teak-producing, trouble-saving, Turko-egyptian, wrath-consumed.
Smiled. Because there’s a** mile ** between the first and last letter. 
(I’ve been saving that one since 1959)
can sense a programmer with free time…and a scripting language like perl to play with. Hmm…
To hijack this thread, I can’t help but mention the word “intuitionistically” that has five "i"s. It is a word that I use a fair amount too, since it comes up in the kind of math I sometimes do. Whenever I write it, I am amused by the number of "i"s. What a nerd!
Is there any other common word that has five or more instances of a single letter. I might mention “deeded”, a six letter word made with just two letters.
Off the top of my head, “indivisibility” (6 i’s) and “inimitability” (with 5 i’s) works.
Ah, there’s a list of somesuch words on this Wikipedia page.
Make them plural and you get another I in each one: indivisibilities (7), inimitabilities (6).
The linux words file contains “possessionlessness” which has 8 i’s.
I was also surprised to find “humuhumunukunukuapuaa” (a type of fish found near Hawaii) which has nine u’s. Not exactly a common word though.
Um, one “i”–eight "s"es. 
On a different (but slightly similar) subject what is the longest monsyllabic word anyone knows of? I got 9 letters : stretched
How about: the longest word with only one vowel. I submit “strengths” (9 letters)
(I think that also matches your record for a monosyllable, scott62)
Oops! :smack: In my defense, I would suggest that “i’s” is visually more aesthetically pleasing than “s’s”.
Would someone please invent the word sparseck to put that joke to rest, please
strenthlessnesses is unique in that it only uses one vowel (e).
Of course, the best 26 letter pangram (a sentence that uses all the letters of the alphabet) is:
MR. JOCK, TV QUIZ PHD, BAGS FEW LYNX.
From the page Scruloose and dtilque linked to:
Sounds like the collected works of Don Martin.