English Grammar 101 and Grammar Monster are two sites that offer online English grammar classes. Both contain free material and allow students to progress at their own rate.
English Grammar 101 offers lessons in punctuation and capitalization, parts of speech, word usage and mechanics. Each subject is broken down into small topics. After a brief explanation, the student completes exercises to reinforce that topic. There are cumulative review tests after each group of lessons.
Grammar Monster offers lessons in punctuation, parts of speech and other grammar topics. It also covers material such as common mistakes in grammar and word usage, and has a glossary of grammatical terms. Each section of topics offers tests with immediate review and explanation.