Name : Standard Notes
Homepage : https://standardnotes.org/ and https://github.com/standardnotes/
Why ? Standard Notes is a famous open-source alternative to Evernote. The most useful point in my opinion is the secured synchronization between multiple devices (PC/Android/iOS) (notes are keeping encrypted). Moreover, it's easy-to-use therefore I reckon it's still in the idea of Solus to be user-friendly.
Open source ? Yes https://github.com/standardnotes/desktop

Hope it's enough. If the answer is no, I will continue to use the AppImage but the integration is far to be perfect.
No matter the answer, thank you for you great job and keep going !