The best resources for checking grammar and punctuation are handbooks and style guides such as Strunk and White's classic "The Elements of Style." Online resources such as Grammarly are also effective. As of 2015, Grammarly users can download the application free of charge.
Grammarly calls itself "the world's best grammar checker," and it is able to identify up to 10 times as many errors as the spell checkers and grammar checkers in standard word-processing programs. Users must create an account, but the service is free to use. Ginger Grammar Checker, another service, offers patent-pending technology to correct grammar mistakes, identify misused words and provide contextual spelling corrections.