Oxford+English+Dictionary

Oxford English Dictionary (www.oed.com)

The Oxford English Dictionary is "widely regarded as the accepted authority on the English language" (oed.com/public/about) and is an indispensable part of any library, particularly at the secondary and college levels. The history and etymology of every word in the language has been exhaustively researched and makes this dictionary a great tool for all classes - English and history, clearly, but also math and the sciences because technological and scientific words are included.

Features:
 *  Advanced search (including by date of origin, subject, and region)
 *  Browse feature, including a browse by category option
 *  Timelines (which show when words were introduced to English)
 *  Sources (which show the top 1000 "creators" of words in English - very interesting!)
 *  Historical thesaurus
 *  Glossary (very helpful for teachers)

Drawbacks:
 * Somewhat pricey ($450/one year)
 * Too complex for many students to use without instruction