A robust, commercial-grade, full-featured, and Open Source cryptography library. [Apache] website

P(ortable|ackageable) NaCl-based crypto library, opinionated and easy to use. [ISC]

An open source, portable, easy to use, readable and flexible SSL library, previously known as PolarSSL. [Apache2] website

A free C++ class library of cryptographic schemes. [Boost] website

An implementation of the TLS/SSL protocols. [Apache]

Small portable AES128/192/256 in C. [PublicDomain]

A fairly comprehensive, modular and portable cryptographic toolkit. [WTFPL]

Library and command line tool to detect SHA-1 collision in a file. [MIT]

A lightweight, secure, easy-to-use crypto library suitable for constrained environments. [ISC]

C++11 header-only message digest (hash) library. [PublicDomain]

A collection of hash functions, ciphers, tools, libraries, and materials related to cryptography.

A library for homomorphic encryption and its applications. [Apache2]

A cross platform file encryption utility. Encrypted files are portable across all supported operating systems and processors. [BSD]

A portable and fast cryptography library. [LGPLv2.1+]

A crypto library for C++. [BSD-2]

A complete and free implementation of the OpenPGP standard. [GPL]

A secure communications library implementing the SSL, TLS and DTLS protocols. [LGPL2.1]

A general purpose cryptographic library originally based on code from GnuPG. [LGPLv2.1+]

A free version of the SSL/TLS protocol forked from OpenSSL in 2014. [?]

A low-level cryptographic library. [LGPL]