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The Internet Grammar of English
Course written primarily for university undergraduates but useful for anyone as it doesn't presume any prior knowledge of grammar. Free for students who log in from a UK university and all explanations are free.
www.ucl.ac.uk/

English Grammar Online
Offers exercises, explanations (grammar, writing), games, literature, teaching materials and information on English-speaking countries
www.ego4u.com/

English Grammar from ThinkQuest
Includes various chosen topics with explanations and examples.
library.thinkquest.org/

English Page
Offers free grammar and vocabulary exercises, topical tutorials, reading exercises, dictionaries, and other resources for intermediate and advanced learners.
www.englishpage.com/

English Quizzes
Interactive, illustrated vocabulary and grammar exercises, rated according to difficulty.
www.prof2000.pt/

Rose of York English Language School
Free grammar lessons.
www.roseofyork.co.uk/

English Grammar
Describes the different parts of a sentence and provides examples of correct and incorrect usage.
www.iscribe.org/

English for Techies
By Christian Lassure, associate professor of English. For French student technologists. Grammar, false friends, translation methods, glossaries. Mechanical production engineering, industrial computing, and computer graphics.
www.english-for-techies.net/

English Exercises EFL Exercises and ESL Quizzes
Exercises and games to test business English, grammar and vocabulary.
www.better-english.com/

Blue Book of Grammar and punctuation
Grammar and punctuation reference guide including exercises with answers.
www.grammarbook.com/