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Using english.com: English Grammar Quizzes & Worksheets
A nice treasure trove of printable grammar worksheets organized by proficiency level: beginner, intermediate, and advanced. Emphasis is on parts of speech but you'll also find some that focus on common spelling issues and vocabulary...
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English Language Activities, Exercises and Tests
This site offers tests and activities which can be used by a teacher as well as by an advanced ESL high school student doing a self-guided study.