Here are some websites that will help both students and teachers of ESL/EFL/TESOL:
Ask the English Teacher
This is the best I've seen yet. It's set up in the form of a blog, but students are encouraged to email this teacher with their questions. The posts are responses to emails.
Dave's ESL Cafe
Click on Stuff for Teachers or Stuff for Students for oodles of wonderful help.
Activities for ESL/EFL Students
There are all kinds of excellent help here, and in an amazing array of languages/English.
ESL Self-study Materials
Click on the different links for a list of print, audio, and computer software materials.
Interesting Things for ESL/EFL Students
Many, many good links here!
ESL CyberListening Lab
Excellent topics for gaining help and experience with verbal English.
How to Write an Essay
Really good ideas here for ESL and English students as they learn to write essays.
English Idioms, Idiomatic Expressions
Learning English and refining vocabulary - learning from others' mistakes.
Grouped by categories
One more website for all purposes ESL!!