Some online grammar help sites that can check a writer's grammar for free include Grammarly, Paper Rater and Ginger Software. These sites will check the grammar of text entered, highlight errors, give suggestions and provide information on grammar rules.
Grammarly provides grammar checking by scanning a writer's text using more than 250 grammar rules. The grammar will also check for words that are spelled correctly but not used in the correct context, such as the use of affect instead of effect.
Paper Rater provides free grammar checking of a writer's text in both American and British English. The site can also scan the writing to detect possible plagiarism.
Ginger Software will detect and correct many common grammar mistakes. The software will also provide suggestions for alternative sentence structures.