OutWiker – free and open source, multi-platform software keeping notes in a tree

Simplenote – free and open-source, simple and cross-platform software for creating notes and to-do lists with the ability to sync across all your devices

Joplin – free and open source cross platform note taking and to-do application with synchronization capabilities

μPad – is a free and open source cross-platform software for creating and organizing notes

Knowte – free and open source, multi-platform software for creating and organizing notes

Standard Notes – free and open source app with fully encrypted notes