Dictionaries:
Cambridge Dictionaries Online: Includes English, Business English, Spanish/English and more
Merriam-Webster Dictionary: Online dictionary includes Word of the Day, Spelling Quizzes, words by topic
Acronym Finder: Definitions for acronyms, initials, abbreviations and codes with an emphasis on military, technical, and social jargon
Phrase Finder: Meanings and origins of thousands of English sayings, phrases, idioms and expressions
Thesauri:
Roget’s International Thesaurus of English Words and Phrases
Thesaurus.com: Thesaurus, dictionary, crossword puzzles, and writing resources
Grammar:
Grammar Bytes!: Terms, rules, exercises, teaching materials and more
Grammar Girl: Fun and informational grammar site; excellent resource for style and word choice clarification
Online English Grammar Resources: Comprehensive online grammar resource includes concept mastery tests
Elements of Style: Full-text version of Strunk's classic reference, The Elements of Style