The New York Times shows a regular feature in their Learning Network specially thought out for English learners: Test Yourself in English, where readers are tested on their knowledge of English grammar, vocabulary and reading skills.
Every week two one-question tests are published, and the archive has run up around 300 tests so far.
Either if you are preparing for an English exam at an intermediate level or above or you want to practise just for fun, Test Yourself in English looks like a good place to do so.