trezor-crypto - Heavily optimized cryptography algorithms for embedded devices.
emmy - Library for zero-knowledge proof based applications (like anonymous credentials).
s2n - Implementation of the TLS/SSL protocols.
LibHydrogen - Lightweight, secure, easy-to-use crypto library suitable for constrained environments.
saltpack - Modern crypto messaging format.
Keybase - Secure groups, files, and chat for everyone.
Signal - Secure messenger.
Cryptocoding - Guidelines for low-level cryptography software.