EnglishStudents.com

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/

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

English Bites
TV show focused on improving English language skills.
abcasiapacific.com/

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

Grammar Online
Offers printable grammar lessons including explanations, exercises, and answers.
online.ohlone.cc.ca.us/

Indiana State University Grammar
Includes information on irregular verbs, modals, plurals, prepositions, tense and aspect, two-word verbs, and article usage.
isu.indstate.edu/

Cats Family Grammar
Offers grammar explanations in English, German and Russian.
www.cats-family.com/

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

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