Note taking, is a daily requirement for most of us, however, finding a good solution that can compete with commercial ones is hard. There is free software with a set of features that can compete and win with a commercial solution.
I choose the notes with advanced and usable features as powerful encryption, cloud backup and synchronization, private note support, and multi-platform as it can run on Windows, Linux, and Mac OSX.
Turtle is an open source note-taking application, built to organize notes, passwords, bookmarks, and images. It works fine with Linux, Mac OSX, and Windows. It has Google Chrome and Firefox browsers extensions.
Turtle has powerful features as client-side encryption, markdown support, tagging, powerful search, sharing, browser extensions with clipboard support, TeX math support, file attachment, and RTL (Right-to-Left) support.
Laverna is a note-taking application offers many features despite it's simple user-interface as note synchronization and backup to Dropbox and remote services, markdown editing, distraction-free mode, todo notes, code highlighting, importing and exporting notes, and powerful encryption for notes.
Laverna is an open source project released under MPL 2.0 license. It works fine on Windows, Mac OSX and Linux. It also provides a self-hosted web version.
Zim is an open source desktop wiki application, Its perfect to take notes and organize them together as connect them to each other. It works same as Wiki but on your desktop which makes it great to draft emails, articles, blog entries, take notes of lectures and meetings.
Zim is supported by many Linux distros and it has a Windows installation package, but unfortunately, it does not work on Mac OSX easily.
TiddlyWiki is a single-file mode wiki application, It's tiny but very powerful, I had been using it over the years, and I can't yet find other alternatives like it. using it is very simple to download the HTML file, write and save the file. That's it.
TiddlyWiki is very rich in options, features, supported tools and environments, and its even developer's friendly. with few tweaks, you can use it as a todo list, effective project management tool and of course writing drafts and notes.
TiddlyWiki has extensions for all the major browsers but more importantly, it works well with Beaker browser over Dat protocol. which means it works for teams.
Joplin is a free open source note program that supports end-to-end encryption, note synchronization to online services and cloud-like NextCloud, WebDAV and OneDrive, importing Evernote notes and file formats, offline mode support, file attachment.
Joplin has been released as a desktop application for Windows, Linux and Mac OSX. Joplin developers also released Android and iOS apps and command-line application as well. Joplin also has accessories like Google Chrome and Firefox extension as Web Clipper to take notes of web pages with simple few clicks.
QOwnNotes is a desktop free open source note application, with the support of OwnCloud and NextCloud. It supports markdown, customizable shortcuts, vim mode, Evernote imports.
QOwnNotes supports note encryption. to keep the notes private, Which is very useful combining with its feature as portable software as it works directly from the USB stick.
QOwnNotes has been released for Linux, Mac OSX and Windows.