The Oxford English Dictionary (OED) is a guide to the meaning, history, and pronunciation of 600,000 words - past and present - from across the English-speaking world. The OED Online also features the Historical Thesaurus of the OED, which arranges the dictionary by meaning.
N.B. The current online version of the OED is derived from the printed second edition of 1989, which is in turn based on the original edition published 1884-1928. A full revision of the OED for its third edition began in the late 1990s. This is a large-scale, long-term undertaking. As of 2012, about one third of entries in the online edition had been updated. The dates of the latest upload are provided for your information. However, bear in mind that not all entries have been fully revised from their original entries (apart from bibliographical standardisation) so it is always worth checking the date information at the foot of the page and the latest date of quotations in the entry to ascertain how old the entry you are viewing may be.
Oxford Reference brings together over 2 million entries from across Oxford's Dictionaries, Encyclopedias, and Companions.
It allows you to search across hundreds of different language dictionaries including American, Australian, British, Canadian and New Zealand English. You can also browse or search many subject specific dictionaries such as Dictionary of Literary Terms, Dictionary of critical theory, Dictionary of Drama.
The Middle English Dictionary offers a comprehensive analysis of lexicon and usage for the period 1100-1500. This electronic version of the MED preserves all the details of the print MED, but is searchable in ways impossible within any print dictionary.
The MED is part of the Middle English Compendium, which has been designed to offer easy access to and interconnectivity between three major Middle English electronic resources: an electronic version of the Middle English Dictionary, a HyperBibliography of Middle English prose and verse, based on the MED bibliographies, and a Corpus of Middle English Prose and Verse, as well as links to an associated network of electronic resources.
The full page image scans of Johnson's Dictionary are available via Eighteenth Century Collections Online. You can also access a transcribed version at http://johnsonsdictionaryonline.com - not all entries have yet been transcribed.
Johnson's Dictionary Online is an easily searchable version of the first edition of Samuel Johnson's Dictionary of the English Language. It allows you to view entires online and see a scan of the original page entry.