On our "Must-Have Utilities" episode of Mac Power Users, I mentioned Grammarly as a tool I’ve been using to enhance my writing. Grammarly is a plug-in for your web browser that will help you common grammar mistakes, spelling errors, punctuation, and more.
The basic version will check for critical grammar and spelling errors. There’s also a premium version available that will perform advanced checks for punctuation, grammar, context, and sentence structure, make vocabulary suggestions, genre-specific writing style checks and has a plagiarism detector.
For Mac users, Grammarly is primarily a web-based grammar tool. They do have a Mac App that you can copy/paste text to check it. Unfortunately, their Microsoft Word plugin is PC only at this point and there is not an iOS App. Supposedly they are working on expanding their feature set. If Grammarly can expand into Word for Mac or plug-in to native Mac Apps it will become much more useful.
For anyone who writes on the web (blogs, etc.) the free version is a no-brainer. Since using it I’ve found a significant reduction in simple typos and incorrect word use. If you write for a living or don’t have someone to double-check your work before it goes out you might want to invest in a premium subscription which starts at $11 a month.