GnuTLS (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...
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.
wolfSSL offers an embedded SSL implementation that is portable, progressive, and easy to use with products including the wolfSSL embedded SSL library and the wolfSSL Embedded Web Server.