Oxford Dictionaries Online is an English language online dictionary. In addition to definitions it includes synonyms and antonyms, sample sentences, audio pronunciations; writing, grammar and spelling guidance.
The Oxford English Dictionary is the authority on the English language. It is a complete guide to the meaning, history, and pronunciation of 600,000 words— past and present—from across the English-speaking world. The history of individual words, and of the language—traced through 3 million quotations, from classic literature and specialist periodicals to films scripts and cookery books is all available in this dictionary.
Grolier Online includes both the Americana Encyclopedia (which is appropriate for high-school students and beyond) and Grolier Multimedia Encyclopedia (which is suitable for middle-school and high-school students).