We brainstormed for a few minutes before turning to Google. Thanks to "Fun With Words," we discovered that dermatoglyphics, misconjugatedly, and uncopyrightable each qualify for the longest English word (at 15 letters) using each letter only once. Several other websites suggest that uncopyrightables would beat them at 16 letters, but there seems to be much debate--among people who have the time/energy to dispute such things--as to whether or not it's a legitimate word.
Here are some other fun words:
Cabbaged and fabaceae are the longest words that can be played on a musical instrument.
Strengths is the longest English word which only contains one vowel. That could be useful in Scrabble, right?
Facetiously, eleven letters long, is the shortest word in the English language that contains all six vowels in alphabetical order.
Want to read some more? Check out Fun With Words.
Do you have a favorite word? I particularly like bamboozled and flabbergasted. And my favorite thing to say to my kids (next to "I love you," of course) is "Well, you're in a pickle, aren't you?"