Answers.com
http://www.answers.com/
Not only a dictionary, but also for searching answers for your questions. You can search for a word, a phrase, a person or even search for the answer such as 'Future trends of educational technology ' .
Thesaurus
http://thesaurus.reference.com/
A dictionary of synonyms and antonyms. Very helpful when writing.
Google dictionary
http://www.google.com.tw/dictionary
