Grammar

English Grammar 101
Extensive collection of lessons covering topics in beginning through advanced English grammar. Online exercises for each lesson provide immediate feedback.

EnglishGrammar.org
Simple, accessible exposition of most English grammar rules, with lessons, exercises, writing guides, and more.

Grammar Glossary
Complete list of grammatical terms and definitions with convenient links to lessons relating to each one.

English for Everyone: English Grammar Guide
Good old-fashioned approach to English grammar. It's hard to find good modern grammar books This one does the job well.

UsingEnglish.com
A mega-site for students and teachers of English grammar at all levels. Includes forums, specialized vocabulary lists, resources, and writing and citation guides.

LearnEnglish Grammar (U.K. Edition)
#1 Apple Store grammar app presenting fun activities with four levels and thousands of questions to sharpen English grammar skills. More from the British Council
here.

Five Grammar Apps for Kids
Scholastic has compiled this short list of apps for kids focusing on parts of speech, punctuation, etc. Find literacy apps for older kids
here.

Sentence Diagramming
This old-school analytical technique trains practitioners to clarify the meaning and structure of sentences. See also
SenGram, a popular sentence diagramming app.


Vocabulary

Vocabulary.com
Best vocabulary development site online. Fabulous dictionary, list creation/editing, and multiple question formats make it fun to learn new words.

Word Power Made Easy
Classic vocabulary development book. “Make words your slaves in 15 minutes a day.” A great way to stimulate and direct a word collection hobby.

Vocabulary Cartoons
Creating outrageous, hilarious pictures or stories is a great way to memorize new words. Additional volumes here and here help train students in this powerful technique.


Dictionaries

Merriam-Webster
Excellent dictionary/thesaurus. “Did You Know” section discusses etymology and word usage. Simplified definitions for kids, foreign English language earners, etc.

Dictionary.com
Basic online dictionary provides definitions, synonyms, word origin, pronunciation, and examples in context.

Thesaurus.com
Companion site to dictionary.com, provides antonyms and a comprehensive list of synonyms sorted by shade of meaning.

wordnik
Multiple definitions with etymologies from major online dictionaries. Includes pictures, comments, and related words (e.g. rhymes).

Online Etymology Dictionary
This resource provides short definitions and extensive information on the history and etymology of words, including related forms and expressions.

OneLook Dictionary Search
Reverse dictionary finds words with a given meaning. Searches dozens of online dictionaries simultaneously (returning a list of links to definitions).

Mnemonic Dictionary
Word lookup providing user-suggested mnemonics (with ratings) to aid memorization, along with basic definitions and synonyms.

Master Root List
Alphabetized list of more than 1000 Latin/Greek roots of English words, with examples. Learning Greek and Latin words is a well-trod path to expanding English vocabulary.


Other

20 AI Writing Tools
Impressive compilation of popular AI grammar checkers and writing assistants. Most cost money; some may be worth it. Check reviews before investing cash.