To prevent myself from asking an obvious, silly question multiple times: What are the English language tools you found most useful?
Please, one tool per answer.
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This is a great tool for finding out how words are actually used in different registers of English, ranging from informal spoken English to formal academic written English. In this answer I used it to find out if the word prepone was used with any regularity in American English (it is not). In this answer, I used it to compare incidences of “an historic” with “a historic”, to see if one is used orders of magnitude more frequently than the other (almost four to one in favor of “a historic”).
It is also useful for researching collocates—which words frequently go with other words. For example, in this answer I used it to compare “on the bus” with “in the bus” (“in the bus” is used sometimes when the bus is stationary). In this answer I used it the part-of-speech searching ability to compare how frequently none was used with a singular and plural verb forms (two to one in favor of plural).
Overall, COCA is a very useful tool for researching how the language is actually used, not only for debunking myths about language, but also for learning something new about how the language works.
Language Log is a collaborative blog about language, most of whose contributors are academic linguists. It is one of the most popular blogs about linguistics, and there are often posts that directly address questions asked here. For example, there were a number of informative posts on singular they I linked to in this answer, and Mark Liberman’s post about the mythical rule forbidding beginning sentences with conjunctions was informative in this answer to the question “Why is it bad to start a sentence with and?”
The Oxford English Dictionary (OED) is a very valuable and rich resource.
When you look up a word, for example, favorite, it provides a comprehensive account of use, history, synonyms, etc.
Note that unlike many free resources in this list, the OED requires a monthly or yearly subscription. However, your library may subscribe and this would allow you to access the OED for free.
It's great for a lot more than just definitions. It's usually the first place I go for looking up etymology, pronunciation, and often derivations/cognates of words.
The cross-connectedness of information in Wiktionary is really what sets it apart and makes it an excellent (if sometimes imperfect) resource.
Google Books NGram Viewer displays a graph showing how those phrases have occurred in a corpus of books over the selected years.
Not Google Books, Language Tools, or even word trends. I mean the search engine. If I am curious about a sentence or spelling, I search for it. If the search returns interesting results similar to what I'm writing about, the sentence was good. If it returns badly-spelled pages about unrelated topics, the sentence is no good.
This book, which can be read for free using Google Books, has a lot of useful usage information that is based on research into how words are actually used (as opposed to how some usage writer would like them to be used). Their commentary was helpful to me in this answer regarding less vs. fewer. It was also useful during my research for this answer regarding usage of the word myself in non-reflexive contexts.
The British National Corpus (BNC) is a 100 million word collection of samples of written and spoken language from a wide range of sources, designed to represent a wide cross-section of current British English, both spoken and written.
Also searchable with a less elaborate interface here.
Etymonline is an online Etymological dictionary, very handy for tracing the origins of words. Unfortunately it tends to be very terse, sometimes to the point of ambiguity.
This is a site for hearing pronunciations of words recorded by “ordinary” people. Many words have multiple recordings in different dialects, and each recording has votes on whether others think it is good or correct.
An invaluable and up-to-date resource for looking up slang and other words that are often absent in conventional dictionaries.
Chicago Manual of Style is really useful, especially looking at the example sentences of correctly-typeset English.
It's a lexical database of English. Nouns, verbs, adjectives and adverbs are grouped into sets of cognitive synonyms (synsets), each expressing a distinct concept. These "synsets" are interlinked. It's more rigorous than a normal Thesaurus, in that it tries to "pin down" every unique meaning of a word rather than just listing a bunch of synonyms with different shades of meaning.
It's a fascinating project. It's an attempt to be thorough and methodical about categorizing semantic meaning in English for use in computational linguistics and natural language processing.
Fowler's Modern English Usage (original book or the second edition edited by Sir Ernest Gowers) is fun to read and educational.
I don't recommend the new edition.
Practical English Usage by Michael Swan is very handy if you need to justify edits to a non-native speaker.
Wordnik.com is an online dictionary and language resource that provides dictionary and thesaurus content, some of it based on print dictionaries such as the Century Dictionary, the American Heritage Dictionary, WordNet, and GCIDE. Wordnik has collected a corpus of billions of words which it uses to display example sentences, allowing it to provide information on a much larger set of words than a typical dictionary.
Google word translation
The translation is displayed in a tooltip after you position the mouse pointer over a word. The Google Toolbar includes this feature.
TheFreeDictionary.com used with the print layout is currently the fastest online dictionary.
It is easy to configure it to be your word search engine in Chrome & Opera so that you don't have to type the entire URL every time you want to search the meaning of a word or phrase.
Update: The dictionary service provided by Vocabulary.com is blazing fast too. It is easy to configure the service to search for word meanings right from the Chrome address bar
This site allows searching of two- to eight-word phrases from the British National Corpus.
Project Gutenberg has tens of thousands of free ebooks. Useful for looking up old and classic texts.
This thread has some really good online resources. It would be convenient if one could reference the bulk of them in just one place: a page of favorite links ~ an online wordsmithery of sorts.
After double-clicking any word you get a quick pop-up definition with an icon to hear the word pronounced. It also translates foreign words and supports the following dictionaries:
The Oxford Advanced Learner's Dictionary, from the publishers of the OED, is one of my favorite dictionaries.
It has entries written using the Oxford 3000 keywords, so they're easy to understand, suitable for learners and experts alike. Each entry includes British and American English audio and an IPA pronunciation key. The example sentences and usage notes are great.
For focusing on American English, the Oxford Advanced American Dictionary is also now available, which includes essentially the same information and features as the American English parts of the OALD entries.