TreeProjects is a notes organizer, a very powerful one, that I became a fan of many years ago. It had started life as the free Smereka and grew to a full featured outliner and organizer. I wrote a brief review of it in 2011. It has many features that made and make it complete like tagging, linking nodes, global search and reminders.
After years of no development Yaroslav has made the last version, 2.9.4, freely available and paying for a licence becomes optional, if you want to help him.