Azar Grammar is a global community of English teachers based in the USA. English teachers, and students for that matter, have at their disposal lots of classroom materials and guides to help them in everything connected with grammar in their everyday job.
The Grammar Speaks section of the site is specially relevant for English students, where talking heads speak directly to them and give short, succint explanations about all grammar points.
This grammar explanations are divided into two parts: Fundamentals of English Grammar Interactive, for the lower levels, and Understanding and Using English Grammar Interactive, for higher levels.
The site is a must for English teachers, and so is the Grammar Speaks section for English learners.