Antagonyms - words that have opposite meanings (e.g., "bad" can mean bad or good depending on usage).
ArtLex - "Welcome to ArtLex. You will find definitions of more than 3,100 terms here, along with numerous illustrations, pronunciation notes, great quotations, and links to other resources on the Web."
bab.la: Online Dictionaries - "Online dictionary for 24 languages.
And much more about languages and translation: Conjugation, Quizzes, Games, Forum, etc."
Ethnologue Database - on-line database of information about 6,500 languages. Includes alternate names, number of speakers, location, dialects, linguistic affiliation, and other sociolinguistic and demographic data.
Fine Dictionary - "FineDictionary.com includes definitions from 4 dictionaries (WordNet, Webster's Revised Unabridged Dictionary, Century Dictionary and Cyclopedia and Chambers's Twentieth Century Dictionary), illustrations, example sentences, interesting acts, quotations, idioms and etymology as well as related words (synonyms, hypernyms, hyponyms)."
Fluency MC - "How to speak English fluently? 34 renowned English learning specialists share their #1 tips for how to speak fluent English."
Freedict.com - translating dictionaries from English to the following languages: Afrikaans, Danish, Dutch, Finnish, French, Hungarian, Indonesian, Italian, Japanese, Latin, Norwegian, Portuguese, Russian, Spanish, Swahili, and Swedish.
Free Online Spell Checker - "This free online spell checker and spell check enables you to check your texts for spelling, grammar, or punctuation mistakes and offers instant feedback to correct spelling errors."
FreeTranslation.com - "FreeTranslation.com is a straight forward, easy-to-use site for rapid translations where you can get the 'gist' of foreign language text and web pages."
GrammarCheck.net - "Now you can grammar and spell check any text or document online. Simply copy and paste your text onto our website to proofread, review, and correct it."
Guide to Grammar and Writing - The Guide to Grammar and Writing contains scores of digital handouts on grammar and English usage, over 170 computer-graded quizzes, recommendations on writing -- from basic problems in subject-verb agreement and the use of articles to exercises in parallel structures and help with argumentative essays, and a way to submit questions about grammar and writing."
LangToLang.com - Free multilingual dictionary offers bidirectional, single and multiple words translation from/to: English, German, French, Spanish, Russian, Italian and Turkish language pairs.
Languages - This BBC site provides resources to help you learn Spanish, French, German, Italian, and other languages through online lessons. You can also learn holiday phrases in 34 languages and more.
Lexical FreeNet - a directory of over 500 translation dictionaries and glossaries.
Literary Encyclopedia and Literary Dictionary - The aim of The Literary Encyclopedia and Literary Dictionary is to provide profiles of the lives and works of literary authors whose works are valued in the English language, and to do so within an electronic publication which will enable readers to explore literary history as never before.
Medical Dictionary - Site features encylopedic dictionary of over 9,000 classic and contemporary medical terms. Written entirely by physicians for both professional and non-professional readers site assists everyone concerned about health -- their health and the health of those that matter to them.
Music Dictionary Online - Our Music Dictionary Online explains words and terms you may meet when reading musical scores, books about music or programme notes on CDs or at concerts. We have made it more comprehensive, by providing copious links to supplementary material in our own Music Theory Online resource, our Musical e-Monographs and external websites.
Newbury House Online Dictionary - This online dictionary contains over 40,000 entries and is based on the best selling Newbury House Dictionary of American English. Providing simple and clear definitions, this online tool provides a wealth of sample sentences, idioms, and a wide array of cultural facts and figures.
Nihongo Master: Learn Japanese Online - "Learn Japanese online with easy lessons, powerful study tools and a great community. Score points and rank against others to become a Nihongo Master."
OneLook Dictionary Search - Look up a word or term in an Internet dictionary or glossary. Free search access to a frequently updated database of words, terms, names, and acronyms.
On-line Chinese Tools - "These pages hope to provide tools to assist people in learning and using the beautiful Chinese language."
Omniglot: A Guide to Writing Systems -This website provides a guide to over 150 different alphabets, syllabaries and other writing systems including a few you will find nowhere else. It also contains details of many of the languages written with those writing systems and links to a wide range of language-related resources, such as fonts, online dictionaries and online language courses.
RhymeDB.com - "What rhymes with Rhyme? - Find all words that rhyme with other words at RhymeDB.com."
RhymeZone - site offers rhymes, synonyms, antonyms, related words, and other phonetic and semantic functions; it also has integrated cross-referencing of famous quotations, Shakespeare, Biblical passages, Mother Goose rhymes, and user-submitted poetry.
Smartling - "Simplify and accelerate translation and localization of your websites, mobile apps, and documents."
Spanish Dictionary - "The world's most popular Spanish translation website. Over 1,000,000 words and phrases. Free. Easy. Accurate."
Specialty Dictionaries - Not even a native speaker knows all the words of his own language. Technical vocabulary is used only by specialists and so is not generally known by non-specialists. At this site you will find dictionaries and glossaries of specialized words in the English language, from Accounting to Wine.
SpellCheck.net - Free online spell checker. Copy and paste a word or document up to 5000 characters to check and correct it online.
TechDictionary.com - Search for thousands of computer and technology terms by term or keyword. Included are; computer, Internet and telecommunications terms, acronyms, emoticons or smilies, chat abbreviations, filename extensions, HTML tags, and domain suffixes.
Tech Terms Computer Dictionary - "The free online dictionary of computer and technology terms. The goal of the Tech Terms Dictionary is to not just define computer terms, but explain them as well. Definitions of computer terms are helpful, but explanations with examples are even better. TechTerms.com contains hundreds of computer and technology terms, all with detailed explanations."
TheFreeDictionary.com - English, Medical, Legal, and Computer Dictionaries, Thesaurus, Encyclopedia, a Literature Reference Library, and a Search Engine all in one!
TheFreeThesaurus.com - "Free Thesaurus is a comprehensive online thesaurus of synonyms, antonyms, and related words. The thesaurus contains more than 145,800 unique entries from three top sources: Collins Thesaurus of the English Language - Complete and Unabridged, The American Heritage Roget's Thesaurus, and WordNet."
Vocabulary.com: Dictionary - "Try the world's fastest, smartest dictionary: Start typing a word and you'll see the definition. Unlike most online dictionaries, we want you to find your word's meaning quickly. We don't care how many ads you see or how many pages you view. In fact, most of the time you'll find the word you are looking for after typing only one or two letters."
Where Do Languages Come From? - "We don't ask ourselves where languages come from because they just seem to be there: French in France, English in England, Chinese in China, Japanese in Japan, and so forth. Yet if we go back only a few thousand years, none of these languages were spoken in their respective countries and indeed none of these languages existed anywhere in the world. Where did they all come from? This question is examined by this site by Exploratorium Magazine."
Your Dictionary.com - "YourDictionary is a free online dictionary that offers definition search from Webster's dictionary for word meanings and thesaurus. Use this dictionary search as a crossword or scrabble dictionary. Also find legal dictionary, technical words, glossary, foreign language translation, medical dictionary, and rhyming dictionary lists."