Duolingo

I am a foreign language teacher and have found Duolingo to be a useful resource in my classroom.  My students sign up at the beginning of the school year/semester and begin with the beginning levels of the site.  If they do not have written homework, they are required to complete a section of grammar on Duolingo.  It helps them to practice outside of class and they tell me that they enjoy the site.  My beginning students find it the most challenging, though my advanced students like the pronunciation part that helps them practice.