http://grammar.ccc.commnet.edu/GRAMMAR/
http://www.glencoe.com/sec/writerschoice/pp/mslessons/grade6/index.shtml
(This one’s fun) http://www.eduplace.com/kids/hme/k_5/proofread/proof.htm
http://www.daltonstate.edu/faculty/mnielsen/writing_lab_practice_exercises_list.htm
http://jtevans.net/tools/passive_territory/
Anything you want to know about grammar, usage/mechanics, writing, style, research...it's ALL here! owl.english.purdue.edu/