Top Alternatives to Notability

Evernote is a note taking application which keeps the notes taken in sync across all platforms using its cloud infrastructure.

Laverna is a JavaScript note taking application with Markdown editor and encryption support.

Notezilla is a sticky notes app for Windows.

Evince is a document viewer for PDF, PostScript, DjVu, TIFF, XPS and DVI formats.

CherryTree is an hierarchical note taking application, featuring rich text and syntax highlighting, storing data in a single xml or sqlite file.

NixNote is an open source client for Evernote.

Alternote is a note-taking app for Mac OS X. Alternote is an alternative Evernote client for Mac with Markdown support.

WizNote is a cross-platform cloud based note-taking client.

GoodReader is an iOS application used to view documents in different formats, including .doc, .pdf, and .xls.

Leanote is a self-hosted note taking app.

Zoho Notebook takes the same catch-all approach as Evernote and OneNote, but has a few design ideas of its own.

Guide is an application that allows you create documents ("guides") which inherently have a tree (which you can modify as you please) and text associated with each node of the tree.

Magpie is a git-backed Evernote replacement.

BeeCanvas is an online whiteboard based visual collaboration tool.

Elephant is a notetaker with a classic interface.

NimbleNotes is a note-taking app.

footnote is a very light and neat Notebook App for elementary OS.