A comprehensive site with explanations and exercises for all levels.
English Grammar Reference and Exercises
More explanations and some exercises.
English Grammar – Rules and Explanations
Explanations and examples; some exercises.
From LCI instructors:
www.englishpage.com (grammar and some vocabulary)
http://daily-english-activities.blogspot.com/ (daily activities)
http://daily-english-activities.blogspot.com/p/more-activities.html (archive of activities)