www.englisch-hilfen.de

Great website with excellent exercises and explanations! Highly recommended!

http://www.perfect-english-grammar.com

 

Also excellent, especially for 2nd and 3rd year students (revision of topics such as reported speech, conditionals, etc.)  

Highly recommended!

http://www.ego4u.com/en/cram-up/grammar

 

... this one's similar to englisch-hilfen.de... Recommended!