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