http://www.learnsite.ch/
http://www.ego4u.com/
http://www.past-simple.com/
Site about all the English tenses with great overviews and summaries.
http://www.englishpage.com
http://www.english-4u.de
http://www.englisch-hilfen.de/exercises_list/alle_grammar.htm
http://www.eclecticenglish.com/
http://www.members.iinet.net.au/~adelegc/index.html