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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A website that provides the definitions of acronyms.
Google Books is useful for searching for real usage and etymology of words and phrases, and for antecedents.
However, care must be taken with metadata, especially when only a snippet is shown: occasionally the book was published later than the the year Google claims it was, and sometimes they accidentally include multiple books for each record.
Therefore it's important to double check the date: scroll up to confirm the real date for "full view" books, and for preview/"snippet view" verify with another source (such as the Internet Archive or Project Gutenberg).
I'm an English as a Second/Foreign Language teacher, and I like to use the Cambridge Dictionaries Online.
It has different levels of definitions from Learner's (which used to be basic or beginner) to Advanced Learner's.
I find it's not only helpful for me when I need to find a way to define a word for a student, but it also helps me understand words I may have never seen before or don't often use. They also have some mobile apps for English students, and a blog that posts about new words in English like lactivism and lets you comment about them.
Chambers Dictionary, famous for its humorous definitions.
An acronym generator generates acronyms from sets of words.
manythings.org is an online "dictionary" which can help you memorize words which are listed according to their frequency.
Word Dynamo from Dictionary.com is a nice way to learn new vocabulary. It has flashcard sets of a variety of different topics.