Dave's ESOL Café
One of the best and most comprehensive site for English Langugage Learners; includes resources for teachers and students

Educational Technology Clearinghouse: Foreign Language Dictionaries
Lists of dictionaries in a variety of foreign languages

Interesting Things for ESL Students
Study site for English Language Learners, includes American slang, reading American signs,etc. The games and exercises are varied and interesting for extra help for ELLs.

UsingEnglish.com
This site is for both teachers and ELLs. It has a good list of idioms, vocabulary, and English grammar terms, and more.

OWL: Online Writing Lab
This is a site loaded with handouts, resources, and exercises to help English Language Learners with basic English words and grammatical concepts.