Two of the most popular grammar checkers are Ginger Grammar Checker and Grammarly.com. Both search for various mistakes with accuracy, detecting errors that are not so obvious. Both services are free, although there is a fee for a premium Grammarly account.
Ginger Grammar Checker searches for contextual mistakes, spelling errors and misused words. Grammarly offers a very similar service, but also alerts writers to significant plagiarism and weak word choices. The goal of both services is to prevent embarrassing mistakes from appearing in the finished product and to aid writers in improving their own ability to spot and correct grammar mistakes.
Both programs can be installed as add-ons and can correct text in all user-written content, including emails, blogs and documents.