Oxford English Dictionary
The OED online.

Oxford Reference Online
Collection of handbooks, dictionaries and encyclopedias from Oxford Univ. Press covering arts, sciences, humanities and social sciences.

Gale Virtual Reference Library
Encyclopedias and reference works on a variety of topics.

Old English Dictionary
Online database consisting of at least one copy of every Old English text, ca. 450-1100.

Oxford Dictionaries Online
English to non-English translations (and vice versa), phrases, some grammar, and more.

Freely Available Resources

A Web of On-Line Dictionaries
Includes a quick lookup in the Merriam-Webster Dictionary, and links to dictionaries in 216 languages.

Cambridge Dictionaries online
Searches Cambridge Dictionary of American English, Cambridge International Dictionary of English, Cambridge International Dictionary of Phrasal Verbs or Cambridge International Dictionary of Idioms.

The WorldWideWeb Acronym and Abbreviation Server

Roget's Thesaurus

Lexical FreeNet Connected Thesaurus
Includes unusual language resources such as a "Rhyme Coercion" thesaurus and a "Related Word" thesaurus.