Readwise - Effortlessly rediscover and organize your Kindle highlights
Encrypted Notepad - Encrypted Notepad is a very simple to use application providing state of the art...
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
Paranoia Text Encryption - Paranoia Text Encryption encrypts text according to a choice of strong algorithms.