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 Language Self-Access Centre
Educational Software Products deals with Computer Aided Language Learning (CALL) software to help students and teachers master the English Language. Their online resource site provides grammar, vocabulary and pronunciation exercises. There is also games, quizzes, idioms, jokes and more. Primarily for students and teachers.

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 English usage. With over 170 computer-graded quizzes integrated with audio clips, it is bound to improve your pronunciation skills.
Primarily for teens & young adults.

HyperGrammar
University of Ottawa has developed this online grammar course for easy and convenient learning of the English Language.

Language and Learning Online
Monash University has a collection of resources to improve your English. These resources include online tutorials, interactive exercises and examples of Writing for Subjects.

Longman English Language Teaching (ELT)
Longman ELT is a leading educational publisher of quality resources for all ages and abilities across the curriculum. Free resources are available for learners and teachers of English language. Try fun online activities, download free support materials and get helpful tips on learning and teaching English. Primarily for students and teachers.

One Stop English
This is a teacher’s resource site organised and run by Macmillan Publishers. The site offers grammar, vocabulary and pronunciation exercises integrated with audio clips and games/activities.
Primarily for students and teachers.

World English
This site contains hundreds of html and javascript-based English language exercises on grammar, vocabulary, reading and writing. It also has links to the best resources available on the web.
Primarily for teens and young adults.