Mathworld by Wolfram
http://mathworld.wolfram.com/
PowerTheasaurus.org
https://www.powerthesaurus.org
Built by writers for writers. The synonymic and other associations are suggested, rated and used by its visitors.
GraphWords.com
https://graphwords.com/
Free English visual dictionary and thesaurus that helps you find the meanings of words and show connections among associated words.
Google NGram Viewer
https://books.google.com/ngrams/
Displays a graph showing how phrases have occurred in a corpus of books (e.g., “British English”, “English Fiction”, “French”) over time.
StackExchange.com
https://english.stackexchange.com/
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts.
