yourDictionary.com
English language dictionary and thesaurus as well as other language dictionaries and speciality dictionaries.
http://www.yourdictionary.com

Merriam-Webster Online
English language dictionary and thesaurus, Spanish-English dictionary, medical dictionary and word games.
http://www.m-w.com

Visuwords
"online graphical dictionary"
http://www.visuwords.com

American Sign Language Browser - from Michigan State University
Contains thousands of video clips for American Sign Language signs.
http://commtechlab.msu.edu/sites/aslweb/

VerbWheel (Spanish)
http://www.vistasonline.com/media/shockwave/inv_VerbWheel.htm

Dictionary.com
English language dictionary and thesaurus, Spanish dictionary, translator, and flashcards (you can make your own, but have to use definitions from general English language dictionary).
http://www.dictionary.com

RhymeBrain
Rhyming dictionary.
http://www.rhymebrain.com