Grammarly, so I'm a bit prejudiced. But then, I do love proper spelling and grammar. As I emphasize in the group I helped found - the Indie Author Group. It's appalling to see how many writers don't know how to do the basics.
It turns out that it matters, as the infographic that Grammarly posted shows and the Huffington post shared.
People with stronger writing skills are better at their jobs and get paid more! Imagine that! *grins*
A reviewer of one of my books was ecstatic that I used the word 'sere' in what was a novel of fairly light content. She was thrilled to find a writer who was actually literate in an unexpected context.
What amazes me sometimes is - despite statistics like these - the number of writers who insist they don't have to follow the 'rules'. My response? It helps to know the rules so you know what rule you're breaking and why. More importantly, will your readers understand what you wanted to say?
I'm also sometimes astonished by the writers who also express themselves poorly in posts or comments (typos I get, text speak only so much). You are representing yourself, if a reader sees a writer post or comment using bad grammar or spelling, what are they going to think. (Unless it's clearly deliberate, as I sometimes do.) Your poor grammar reflects on you. A recent post by a well-known politician in which he - a supposedly noted scholar - demonstrates that he doesn't know the difference between your and you're, and the subsequent Twitter and Facebook feedback, demonstrated this pretty clearly.
It makes even less sense when you can have Grammarly check your grammar and spelling as you go - for free!