Purdue University's Online Writing Lab (OWL) has an abundance of handouts on specific grammar principles. They also have some interactive exercises and a list of online resources that can be helpful to ESL students.
UsingEnglish.com has handouts for ESL students on various principles of English grammar.
Dave's ESL Cafe http://www.eslcafe.com has a for-students section including help with idioms, phrasal verbs, and slang and has online practice quizzes and a student forum.
1-language.com has some good grammar quizzes as well as games and other resources.