Cambridge Dictionaries Online includes examples of usage and mentions differences between British and American usages and spellings.
Dictionary.com Advantage: Each entry lists definitions from several dictionaries. Tabs for Thesaurus, Encyclopedia, Reference and Web. Good resource for finding a substitute for a phrasal verb, for example: to get a hold of = to obtain
Merriam-Webster The Gold Standard: Type in your word and decide whether you want the dictionary or the thesaurus.
Yourdictionary.com Select ”Other Dictionaries” from the top menu to find a section with Grammar Guides, which leads to “English Grammar Rules and Usage.” Scroll down; there’s a lot there.
Lexicool.com is definitely a cool dictionary search module. Take a look:
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Style Guides and Manuals
The Chicago Manual of Style. You can’t live without it, or at least some of us can’t make a living without it!
The Blue Book of Grammar and Punctuation. Free access to some of the sections; nice layout; good review of and easy access to punctuation issues. Page on “effective writing” provides an introduction to the ground rules that will help improve your skills. Also has a page with quick reference explaining those confusing pairs: that vs. which; then vs. than; etc.
The table of contents alone tells you everything you need to know to write well. Worth browsing just to distinguish your strengths from your weaknesses. Note that the links on the Webpages of The Literary Encyclopedia open by clicking on the asterisks.
Research and Documentation Online. An online version of a Diana Hacker Booklet. A student-oriented site with comprehensive resources and explanations for those learning the art of research and documentation.
An exhaustive alphabetized list for easy reference.
Acronym Finder. A helpful tool.
Idiomizer. Search for idioms and their equivalents in multiple languages. It functions as a Wiki site, so sign up and contribute. It’s fun!
Use idioms to give flair and flavor to your writing
Dr. Wheeler’s Website for college students is a useful resource. For a taste, take a look at ”What is Style?”
Online Etymology Dictionary. Alphabetized list includes prefixes and suffixes. Pure fun and enrichment!
Forvo is all the words in the world–pronounced. Use the search box, get acquainted with the tabs or join and help in the effort!
Translation on Wikipedia: a truly fascinating article. I recommend the sections titled “Fidelity vs. transparency” and “Equivalence.”