My Top Favorites
These are sites that I go back to over and over again!
More Great Resources
This site has a wide variety of topics; you have to search through for the grammar point you want, though.
This site is so much more than quizzes--it has is really thorough with great explanations and tons of exercises.
Learn English Feel Good
This website may not have explanations, but such a wealth of exercises--great for when your students need extra practice--great upper level resource!
Even if you don't use the Azar grammar books, this site has a lot of useful extras: powerpoints, song activities, and extra handouts.
English 4 U
Many, many online exercises, graded in easy, medium or difficult.
Road to Grammar
Nice supplementary quizzes on a wide variety of grammar issues, also mixed in with vocabulary work and functions.
Nice links to exercises on other websites based on verb tense
Gerunds and Infinitives
About 10 exercises to practice these.