ESL Blues
Lots of games and quizzes. Mostly grammar but also vocabulary and reading. Take one of the diagnostic quizzes to find out your level. The level of difficulty for each activity is indicated using a number code.

Activities for Esl Students
Mostly grammar and vocabulary. Lots of good quizzes and games sorted by level. Do the easy ones and work your way up!

English Club (Grammar Page)
Excellent website with explanations and activities on every aspect of English learning and teaching (grammar, vocabulary, listening, business English, test taking, reading, pronunciation, etc.).

The Purdue Online Writing Lab
An excellent resource from Purdue University with sections on general, professional and academic writing as well as grammar, style and formatting. Good for intermediate and advanced learners as well as native speakers.

Learn English feel Good (Grammar Page)
Lots of online grammar quizzes.

English Grammar Secrets
Part of the Better English Lessons website produced by English teachers Caroline and Pearson Brown. Emphasis on verbal forms.

Grammar Bytes
Definitions of grammatical terms, grammar lessons, exercises and more!

HyperGrammar
A complete review of English grammar. Very complete. Best for advanced students and native speakers.