Aside from all the other online dictionaries, Scrabble players may be interested in the following sites:
Hasbro’s word lists for tough times (including Q without U, two letter words, X words, and more). Wordplays.com’s tools for word games is a collection of web apps that would be handy to use (if it were legal to do so) during a game. Mark has developed a number of word lists and other scrabble tools. And we can’t ignore the word lists from the Internet Scrabble Club or the ENABLE dictionary.
Palm users may be interested in Scrabble for Palm and the ENABLE Scrabble dictionary for Palm.