Search the American Heritage Dictionary of the English Language, Roget's Thesaurus and other sources. Also useful for translating text.
The Middle English Compendium contains three Middle English electronic resources: the Middle English Dictionary, a Bibliography of Middle English prose and verse, and a Corpus of Middle English Prose and Verse.
Electronic version of the twenty-volume The Oxford English Dictionary, Second Edition and three-volume Additions Series.
Contains 100+ dictionary, language reference, and subject reference works full text and searchable.