Grammar Monster is an interactive website designed to revise (or teach) English grammar and punctuation. It is aimed at native English speakers and advanced students who are required to produce high-quality correspondence and want to improve their existing accuracy.
Although the site has paid-for items, the majority of what you see if free, including lessons and test on punctuation, definitions of parts of speech, grammatical terms, a section on common grammatical errors and also easily confused words.
Grammar monster is also suitable for ADVANCED ESL students. Grammar Monster is designed for people who already write reasonably well and doesn't cover basic grammar points which a native English speaker would already know. For example, the site shows how to use semicolons, but not how to conjugate verbs.