This website takes a fun approach to using English for ESL students. They can connect with other ESL students and especially fun are the idiom and slang sections.
http://www.eslcafe.com

This is an interactive grammar review called Grammar Bytes.
http://www.chompchomp.com

A Home Page offers another list of links for English as a Second Language students and teachers.
http://www.rong-chang.com

Grammar Safari is an ESL activity site that uses real language on the internet.
http://www.iei.illinois.edu/grammarsafari/grammarsafari.html

This site has grammar help on the word level, the sentence level, the paragraph level, and the essay level. It includes power point presentations on some lessons, explanations, practice and quizzes. 
http://grammar.ccc.commnet.edu/grammar/textonly.htm

This page works on popular tricky problems involving tense, prepositions, vocabulary, irregular verbs, etc.
http://www.englishpage.com