Turtl - The secure, collaborative notebook
UpNote - A beautiful and clean cross-platform app for notes organizer
Tomboy - Apps/Tomboy - GNOME Wiki!
Omni Notes - Note taking open-source application aimed to have both a simple interface but keeping smart behavior
Carnet - Opensource rich text note taking app
Cryptee - Cryptee is a safety and privacy focused, encrypted and cross-platform personal data storage service. You can write personal documents, notes, journals, store photos and all sorts of other files.