Scholastic's Word Wizard Dictionary is an excellent resource for teachers and students alike. The dictionary provides simple definitions with audio pronunciation of words, hyperlinked synonyms and even pictures in some cases.
But that's not all because there are also two vocabulary tools for teachers: a Quiz Maker and a Mini-Dictionary Maker. You just enter the words, select the definitions you want to appear and hey presto!, you've got your printable quiz or mini-dictionary. Very useful for producing glossaries for CNN video reports, for example.