ESL Blues has activities and quizzes for English language learners. Most of the quizzes are about vocabulary, grammar and usage. The new site has been modified to work on smart phones, and the author says he’s working updating the activities on the old site, too.
If you’re not sure of your level, there’s a page of quizzes to help you check.
The quizzes are interactive, and so if you get the answer wrong, you’ll get a simple explanation:
Another quiz that may be particularly interesting to people who are studying Chinese, too: about common errors made by Chinese speakers.