English Page: This site offers mini tutorials and many grammar quizzes and lessons for intermediate learners.
ESL Lounge: A comprehensive website that offers test-prep, listening and reading practice, as well as some grammar exercises. A good place to start is the advanced level exercises, as the intermediate are fairly simple.
LEO Network (Learn English Online): When you know exactly which grammar skills you need to work on (such as the Oxford Practice Grammar, listed below), you can use the CTL + F to find exactly what you need. Includes exercises and corrections.
Intermediate and Advanced Level
Dave’s ESL Café: Hundreds of explanations for many different grammar points, although you do need to know what skill you want to work on. At the end, take some of the quizzes to ensure you’ve understood.
Oxford Practice Grammar: Take a test to find out what grammar you need to practice. Choose the Intermediate or Advanced level to have a more accurate idea to what this site can offer you.
Study Zone: Choose your level (Upper Intermediate or Advanced recommended) and you will have access to reading, vocabulary, and some grammar exercise.
Guide to Grammar and Writing: When you’re in search for specific pages on grammar points and essay structure (down to the sentence level), look no further! You can pick through various pages to find exactly what you need. Some pages also feature quizzes to help you practice what you just learned.
The Purdue Online Writing Lab: To polish your writing abilities, visit the Online Writing Lab (OWL) to learn new writing and style techniques, as well as some grammar points.