Keepmark is described as 'desktop, privacy first documents organizer. You can organize with it your files, email, bookmarks, tasks, news, and other resources. Its available for Windows, Linux and Mac' and is a Note-taking Tool in the Office & Productivity category. There are more than 100 alternatives to Keepmark for a variety of platforms, including Windows, Mac, Android, Linux and Online / Web-based. Most alternatives are Note-taking Tools and Todo List Managers. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Microsoft OneNote. Other great apps like Keepmark are Evernote (Freemium), CherryTree (Free, Open Source), Zim (Free, Open Source) and NoteLedge (Freemium).
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, can be copied, tagged and modified either from the applications directly or from your own text editor.
Polar is a powerful desktop app for managing all your web content, books, and notes. Polar keeps all your content in one place, supports tagging, annotation, highlighting and keeps track of your reading progress.