http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/index.htm

http://www.ego4u.com/en/cram-up/grammar

http://usuarios.lycos.es/englishweb/grammar.html#GRAMMAR_PRACTICE (Ideal for beginners. Elementary level)

http://www.autoenglish.org/generalgrammar.htm (Ideal for intermediate students and to revise grammar)

http://esl.about.com/od/advancedvocabulary/Advanced_Level_English_Vocabulary_Resources.htm

http://www.wikihow.com/Spell-Commonly-Misspelled-Words

http://eslbee.com/cgi-bin/quiztest.cgi (exercise on subject_verb agreement)