If you feel like tossing off a quick blog post, personal essay or anonymous rant and don't want to set up a whole new blog on something like Wordpress, IO offers a simple an elegant solution to publish your writing online, no account required.
The draw for many will simply be that you don't need to register your email or anything, and you can still publish immediately to their site. That's really useful if you're on something like Twitter and need a little more room to express your thoughts. You can add your name to your post, choose to remain anonymous or connect your Twitter account. You can see the above post here, for example.
IO also has a deceptively barebones interface that hides a few pretty cool features and robust formatting similar to most WYSIWYG interfaces. Moreover, they offer a "Hemingway" tool that highlights adverbs and passive voice to help keep your writing lucid and to the point. There's also a cute option to play ambient background noise while you're writing, which includes stormy weather, cafe sounds and a crackling bonfire. You shouldn't expect this to be a platform where your writing will live forever; rather, it's a place to post text that you want to share now, without the hassle of creating a blog.