The best open source alternative to KeepNote is Notesnook. If that doesn't suit you, our users have ranked more than 100 alternatives to KeepNote and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to KeepNote are Zim, CherryTree, Trilium Notes and Tomboy.
Offers open-source, end-to-end encrypted notes with device-based privacy, cross-platform support, private vault, secure sync, offline access, markdown editor, exports to multiple formats, automatic backups, organizational tools, and strong encryption.
Zim brings the concept of a wiki to your desktop. Store information, link pages and edit with WYSISYG markup or directly typing some lightweight markup syntax and see it previewed as-you-type. Creating a new page is easy by either clicking on the "Create new note"...
Hierarchical note-taking software that is free, open-source, cross-platform. Features include rich text, syntax highlighting, multi-level lists, embedded files, customizable interface, code execution, PDF export, spell check, images & tables, and comprehensive project management support.
The original trilium is transfered to this repo by its owner and the app continues to develop here as stated here: https://github.com/TriliumNext/Trilium
Tomboy is a desktop note-taking application for Linux, Unix, Windows, and Mac OS X. Simple and easy to use, but with potential to help you organize the ideas and information you deal with every day.
RedNotebook is a graphical diary and journal helping you keep track of notes and thoughts. It includes a calendar navigation, customizable templates, export functionality and word clouds. You can also format, tag and search your entries.
Opensource, developement not stalled
wikidPad is a Wiki-like notebook for storing your thoughts, ideas, todo lists, contacts, or anything else you can think of to write down. What makes wikidPad different from other notepad applications is the ease with which you can cross-link your information.
Note taking open-source application aimed to have both a simple interface but keeping smart behavior.
KeyNote NF is a flexible, multi-featured tabbed notebook, based on Windows standard RichEdit control. It's always accessible with a single keypress, even if you work in another application.
This works very much like KeepNote. More than others I've tried.
A simplified, RedNotebook-type journal/diary app. With a focus on minimal design and a distraction free writing environment. Makes a folder full of dated text files that are easy to access even without the journal program.
Notational Velocity is a mouseless application for storing and retrieving notes.
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This is currently the best alternative to KeepNote, lots of features, including rich text.