Obsidian.md - A second brain, for you, forever. Obsidian is a powerful knowledge base that works on top of a local folder of plain text Markdown files.
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
TiddlyRoam - An open source non-hierarchical notebook.
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.
Raneto - Open source, flat-file Knowledgebase platform that uses static Markdown files