GNU TLS
An Open Source implementation of TLS 1.0 Internet protocol as described in RFC2246. [GPL]

Java Secure Socket Extension
Sun's official extension for SSL communication in Java. API allows third party library being used as a provider.

MatrixSSL - Open Source Embedded SSL
MatrixSSL is an open source embedded SSL implementation under 50K. It is designed for small footprint devices and applications requiring low connection overhead.

Network Security Services libraries
NSS implements SSL v2 and v3, TLS, PKCS#5, PKCS#7, PKCS#11, PKCS#12, S/MIME, X.509v3 certificates, and other security standards. These libraries are used by Mozilla / Netscape Communicator and server products. [Open Source]

Open SSL
Collaborative effort to develop a full-featured, and Open Source toolkit implementing the secure sockets layer (SSL v2/v3) and transport layer security (TLS v1) protocols.

RSA BSAFE
It includes everything needed for delivering SSL-enabled applications developed in C, C++, or Java. It is not subject to US export control. [Commercial] (RSA Data Security)

SecureBlackbox
Delphi/Kylix component and ActiveX/DLL library [Commercial]

SSL Library
Java Library. Implements SSLv3 and backward compatible SSLv2. [Open Source]

Yet Another SSL
The yaSSL software package is an open source, dual licensed implementation of SSL. It includes SSL client libraries and an SSL server implementation.