Yellow Level LAEO - Once students pass through the Red and Blue Levels they are ready for more difficult lessons in grammar.
ESL Connect - This collection of links has just about everything you can think of that's related to learning English. This website is especially nice for kids.
English - For intermediate and advanced students.
Speak English with a robot and do other interesting things on this
website. Pretty cool.
Green Level LAEO - The focus here is almost entirely on the passive voice, but students must first complete all of the lessons in the Blue, Red, and Yellow levels.
A website with good resources and links. Geared more towards intermediate
and advanced students.
Page - Good examples and easy to use.
Learner - Lots of free lessons and quizzes.
This is a really great website for developing listening skills
chompchomp - This well-organized and interesting website is worth checking out, but it's not for beginners.
World-English - There's a little bit of everything here.