Website Review: perfect-english-grammar.com
In short: Wonderful explanations of English grammar and nice practice exercises too. Seonaid—the site’s author—covers 95% of all the English grammar you’ll ever need. (Articles are the big missing topic.) Plus, she seems ready to add topics at your request. The site is well organized with links between the explanations and relevant practice exercises.
For students: This is a great site to go to and do easy practice exercises.
For teachers: You might assign your students to read about a topic before you introduce the topic in class. They might not have perfect understanding from studying it here on their own, but they would be super-prepped for your classroom instruction. You could use this site as a springboard to less structured practice (like dialogue writing or conversation).