Pro Writing Aid
I highly highly highly recommend this tool for writers! It doesn't matter what you write, this program is invaluable! You can pay monthly or buy the lifetime. Prices are below!
You can do monthly for $20.
Yearly for $120.
Or lifetime (with updates) for $399.
You can get a discount for the lifetime from nanowrimo.org by claiming you wrote 50k words in November. If you've ever claimed to have done this (or actually done it) you have access to a bunch of discounts for different writing programs. The below is a quote from the website with the specific offer.
"NaNoWriMo participants save 40% on one year of ProWritingAid Premium and 50% on one year of ProWriting Academy!"
Keep an eye on Nanowrimo, for a while they were offering $100 off the lifetime option. As you can guess from the above, the website for the program also offers classes, lectures, and articles to help improve your writing. Specifically, below are some of the features.
"Our grammar and style checker contains over 3,000 explanations and videos written by our experts, so you don't need to remember all those crazy rules. ProWritingAid delivers thousands of grammar and style improvements with the touch of a button. Get expert writing advice customized to what you’re writing where and when you need it. Whether you’re writing a new cover letter, a fantasy epic, or a college admissions essay, we’ve got the expert advice you need to make it unforgettable. Our roster of writing experts includes college professors from top-ranked universities, #1 NY Times bestselling authors, industry-leading professionals, and more."
What's nice is that there is a plug-in to allow it to work with most internet browsers, this also works for Microsoft programs, Wattpad, Mac, Openoffice, Scrivner, Rich Text, HTML, and Markdown.
Personally, I love this and I chose the lifetime simply because of the overall savings.