The following pages explain many different aspects of grammar and contain plenty of exercises for you to practice:

Grammar Explained and Practice

The English Club grammar page offers lessons that combine explanation and exercises. Very useful resource if you want to review an aspect of grammar in depth or just want to do some exercises on it. 
http://www.englishclub.com/grammar/index.htm

Grammar quizzes on lots of different topics – if you know your grammar just want to practice, this is a very useful site. 
http://www.britishcouncil.org/learnenglish-central-grammar-grammar-games-archive.htm

More exercises on this German site (no worries, all the relevant text is in English). http://www.englisch-hilfen.de/en/exercises_list/alle_grammar.htm

Gap-filling and multiple choice about lots of grammar topics.
http://www.smic.be/smic5022/exercisesgrammar.htm

Another site with quizzes on grammar and vocabulary.
http://a4esl.org/

A site that offers explanations on learner’s typical grammar problems. Choose the area you are interested in from the list of contents. The explanations are easy to understand, but more examples and less ads would be helpful. http://www.perfectyourenglish.com/grammar/english-grammar.htm

Testing your Grammar

This page offers an online diagnostic grammar test. The explanations are not brilliant, but they point you towards chapters in Murphy’s English Grammar in Use. A useful page to give you a rough idea what you might need to practise. For a more detailed analysis, make an appointment with ELS and we are happy to point you into the right direction.
http://www.world-english.org/diagnostic_grammar.htm

Grammars for Reference

Confused by all the linguistic terminology used on these pages? Try the glossary on this page that explains what words like ‘participal’, ‘determiner’ and ‘conditional’ mean.
http://www.usingenglish.com/glossary.html

An online reference grammar. You need to know words like ‘verb’, ‘possessive pronoun’ etc. to find your way around, and it does not offer a lot of explanation, but it is good for quick reference and tables of forms.
http://www.edufind.com/english/grammar/toc.cfm

A page that gives very good explanations for tricky areas of grammar. The use of linguistic terminology is sometimes quite difficult, but its ‘how to use’ sections give good advice for advanced language learners and native speakers. http://www.arts.uottawa.ca/writcent/hypergrammar/

A page written for people who want to know how the English language works. It provides clear explanations of linguistic terms, such as ‘determiner’ or ‘clause’. Well suited for all those who want to get to know the terminology, but not an instant help for learners. 
http://www.ucl.ac.uk/internet-grammar/