Top 3 Open-Source Alternatives to Kontact - KNotes

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.

Trilium Notes is a hierarchical note taking application.

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.