OpenSSL is a framework which implements the secure socket layer. It ships with a program/library which gives you access to all those functions.

GnuTLS is described as '(the GNU Transport Layer Security Library) is a free software implementation of the SSL, TLS and DTLS protocols. It offers an application programming interface (API) for applications to enable secure communication over their network transport layer, as well as interfaces' and is a cryptography library in the development category. There are eight alternatives to GnuTLS for a variety of platforms, including Windows, Linux, Mac, BSD and Android apps. The best GnuTLS alternative is OpenSSL, which is both free and Open Source. Other great apps like GnuTLS are LibreSSL, mbed TLS, Themis and Network Security Services.
OpenSSL is a framework which implements the secure socket layer. It ships with a program/library which gives you access to all those functions.

LibreSSL is a version of the TLS/crypto stack forked from OpenSSL in 2014, with goals of modernizing the codebase, improving security, and applying best practice development processes.
Mbed TLS (formerly known as PolarSSL) makes it trivially easy for developers to include cryptographic and SSL/TLS capabilities in their (embedded) products, facilitating this functionality with a minimal coding footprint.
Network Security Services (NSS) comprises a set of libraries designed to support cross-platform development of security-enabled client and server applications with an option support for hardware SSL acceleration on the server side and hardware smart cards on the client side.
Software components for Data Protection, Secure Storage, and Secure Transfer. Trusted in high security, mission-critical applications for decades.
The SSL/TLS and Certificate tutorial for Windows enables you to fast track your understanding of the SSL/TLS protocol, Certificate Management, and secure communication. The download includes many C source code examples, including a secure WebSocket tutorial.

wolfSSL (formerly CyaSSL) is a small, fast, portable implementation of TLS/SSL for embedded devices to the cloud. http://www.wolfssl.com.