It's called Hemingway Editor (hemingwayapp.com). You can either copy and paste text from somewhere into the site, or else type in right in there and it will tell you how terrible you're doing. Or how great your writing is, assuming you are good at that sort of thing. It highlights sentences that are hard to read, very hard to read, all of your adverbs, whenever you're using the passive voice, and the overall grade level of your writing. You know, it's like having Hemingway critique your work.
I'm writing this post first in the app and so far it's telling me that my sentences are all hard or very hard to read, and that I'm using a bunch of words that it thinks I could do better with. Also this is written at the 8th grade level.
I wonder what happens if you paste some Faulkner into this thing?
So there you go. A new bookish thing to play around with. Enjoy.