The Free Dictionary
Hear how a word sounds with this dictionary.  A click of the mouse loads audio clips with no lag time.  You can play electronic scrabble, hang man, spelling bee and word matchups.  www.thefreedictionary.com

Math Dictionary For Kids
Penguins lead the way in this sweetly designed math dictionary for kids.  It includes more than 600 math terms.  Fun is provided by corresponding quizzes and animated examples, like a pelican's beak that opens at different angles-depending on the value entered - to catch a fish.
www.teachers.ash.org.au/jeather/maths/dictionary.html

Merriam-Webster
Word games and an exceptionally nice visual dictionary can be accessed at the top of the home page.
www.merriam-webster.com

Word Central
Offers word puzzles and a build-your-own dictionary section.  Daily Buzzword is good for vocabulary-building exercises.
www.wordcentral.com

Dictionary of Canadian Biography
For senior students or teachers, this U of T/Universite Laval project provides a fully vetted, in-depth resource.  Hosted by Library and Archives Canada, it's easy to navigate and the search mechanism works well.  It is limited to people who died before 1930.
www.biographi.ca

Cambridge Dictionaries Online
This site is simple to use with clear and concise definitions.  Phonetics are neatly tucked under the Show Phonetics button.  Spell a word incorrectly and you are given lots of options.  

www.dictionary.cambridge.org

Your Dictionary
This site provides all kinds of resources.  Look up an obscure word, translate between languages, learn to write your name in 14 languages, play word games or convert currencies.
www.yourdictionary.com

The Visual Dictionary
Simple word searches lead to layer upon layer of meanings with large, clearly presented visuals.
www.infovisual.info

For many more English sites on this and other topics, visit www.oct.ca - Professionally Speaking - netwatch

For French sites
 visit www.oeeo.ca - Pour parler professiona - Cyberespace

Thanks to Lynda Scarrow, the Ontario College of Teachers' web editor, for this information found in Professionally Speaking:  The Magazine of the Ontario College of Teachers, March 2008, p.64.