About.Com: English as Second Language
A comprehensive compilation of articles and exercises on grammar, vocabulary and pronunciation integrated with audio clips, tests and quizzes. Primarily for teens, young and working adults.

BBC Learning English
This site features grammar, vocabulary and pronunciation exercises integrated with audio and video clips, quizzes, games and message boards for a highly interactive learning experience. Primarily for young and working adults.

British Council Learn English
This site basically caters to everyone with different needs. Apart from exercises and games, it also features stories, songs, magazines, cartoons, e-cards and more to enhance your learning.

English Pronunciation
Okanagan University College has developed this useful site and devised units to aid English Language learners in their pronunciation. It features audio and video clips as well as downloadable pdf files.

Guide to Grammar & Writing
The Guide to Grammar and Writing is sponsored by the Capital Community College Foundation. It contains scores of digital handouts on grammar and correct Engilsh usage. With over 170 computer-graded quizzes integrated with audio clips, it is bound to improve your pronunciation skills.
Primarily for teens and young adults.

Phonetics
Pronunciation guides and dictionaries with instant mouseover sounds in English and nine other languages.