Currently using Joplin. It is amazing, and a quick look at Quiver yields a very similar look. I don't know of any implementation that allows "sharing" notebooks like I saw in a screenshot of Quiver, but otherwise, Notebooks with sub-notebook organization, Markdown is possible. Also, there seem to be a lot more methods for accessing Joplin on different platforms, with Windows/MacOS/Linux/Android/iOS/Terminal implementations with DropBox and other file storage services. Highly recommended.
Also, Joplin is FREE. Major bonus.
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