A normative online comprehensive online grammar with clear explanations and examples.
http://www.edufind.com/english/grammar/

Netgrammar is a free online grammar course for intermediate students. Each unit includes grammar, reading, listening and writing activities. It can be used for individual or classroom study.
http://busboy.sped.ukans.edu/~allenq/netgrammar/
main.html

Twenty-four-hour help for ESL/EFL students from an international team of EFL/ESL teachers.
http://www.eslcafe.com/help/

A huge body of exercises here, assembled as a result of diligent co-ordination, effort, and innovation by Charles Kelly.
http://a4esl.org/q/h/

Hints, rules, and exercises on English grammar from
Ruth Vilmi. Highly recommended.
http://www.ruthvilmi.net/hut/help/grammar_help/

A wealth of grammar, vocabulary, collocation exercises and Business English crosswords. 10 mini lessons free sent via e-mail with no obligation to buy. Worth visiting.
http://www.better-english.com/exerciselist.html