Readwise - Effortlessly rediscover and organize your Kindle highlights
Milanote - Milanote is a note taking app for creative work.
Klib - Kindle & iBooks Highlights Manager
CryptoTE - CryptoTE is a text editor with integrated strong cryptography.
Clippings.io - Organize the notes you make on your Kindle
Notebook PEA - Protects your private notes with a password.