Draft, found at Draftin.com, is a great writing web app that I've used a bit but need to find a place for in my writing workflow because it's just such a great interface to write in.
With support for version control (the ability to go back to a previous version of your document), Markdown support, exporting to HTML, Markdown/text, Word .docx and Kindle .mobi it's a robust editor that you can use anywhere. Plus collaborative tools that enable you to share it with someone else for editing/co-writing as well as the ability to hire a pro copy-editor right within the app to improve your work make Draft all kinds of awesome1.
Nathan Kontny, the developer of Draft, recently updated it with a new feature he's calling "Hemmingyway Mode" which, when enabled:
Draft will turn off your ability to delete anything in your document. You can only write at the end of what you've already written. You can't go back; only forward. To return to normal mode, use the same shortcut to turn Hemingway Mode off.
Another feature that's new to me, but I think it's been in Draft for awhile, is the daily quota which allows you to set a daily quota for yourself of how many words you would like to write each day - and Drafts can email you a reminder each day to motivate/annoy you to write.
Draft is free to use and has an option to register and support Nathan's development of Draft for $3.99/month or $39.99/year.
It's worth checking out Draft if for no other reason than to see how a modern web application can disappear into the browser and feel like a native application.