www.thefreedictionary.com/
This website draws on dictionary content from several sources. Its interactive features can be used by high-level students or independent learners. There is also a powerful dictionary search engine that is valuable for learners and teachers alike.
On the main page there are a couple of useful interactive tools for you to try; my favourite is the Spelling Bee. This is an excellent tool for learners of all levels to practice their listening, and work on associating sounds with groups of letters—not always easy in English. The fact that it uses less common words is useful as it will help students to spot the patterns in English spelling rather than just to spell familiar words.
The main page also offers a good selection of simple reading articles, and Quote of the Day gives good topic-related work for class study and reading.
This website is best at providing dictionary reference work. To do this, click on one of the dictionary titles from the What’s Inside selection. There are several specialized dictionaries for technical terms, but a good place to start for simple definitions is the Dictionary/Thesaurus section and put your word in the search box.
The definitions here, provided by the American Heritage Dictionary, are accompanied by recordings, and give a useful synonyms and antonyms box for further exploration of connected words.
A really clever way to use this site is to use the Idioms dictionary. Here you can copy a whole phrase or simply a word into the search box. Try putting in a whole paragraph from a new text or Quote of the Day. The dictionary will help you discover new collocations, phrases and phrasal verbs.
The idiom definitions, provided by the Cambridge International Dictionary of Idioms, give sample sentences, as well as links to other connected phrases. This is a good way to spot connections between phrasal verbs and other idioms for English learners.
Disclaimer: These are ideas for websites to use in the classroom. All sites were active and working as of February 2006, but do check them beforehand. EF is not responsible for the contents on any of these websites. Websites should be quoted as any other outside reference source with a title, author and publication/download date.