There are hundreds of web sites that focus on providing information and quizzes for English language learners. I have spent more time developing my own site than exploring other sites. So I have included only four in this list.
I am happy to be sent recommendations to other ESL portals that learners have found useful.
Links to hundreds of different activities and things to read. Highly recommended!
Quizzes, tests, exercises and puzzles to help you learn English as a Second Language. This project of The Internet TESL Journal has thousands of contributions by many teachers. (Text from the activities homepage.)
This is an outstanding site for learning English grammar and vocabulary. It is the best I have seen so far. The home page is about How to learn English as an ESL student. Excellent!
Another ESL portal with hundreds of activities and other resources.
A nice site with lots of useful information and quizzes for younger learners.