ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
PGP Everywhere - PGP Everywhere is an iPhone / iPad app that allows you to utilize the security of PGP encryption anywhere you type, in almost any app!
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Homomorphic encryption - Homomorphic encryption by IBM allows you to perform computations on its encrypted data without first decrypting it.
Skizzle Email - Skizzle is the most secure way to share end-to-end encrypted files through email, backed by blockchain
KETS Quantum Key Distribution - KETS Quantum Key Distribution service is a utility that allows future-proof, quantum-safe communications at ultra-low power in a low-cost Quantum Key Distribution chip.