OpenSSL : The premier SSL/TLS encryption library
The OpenSSL Project is a collaborative effort to develop a robust, commercial-grade, full-featured, and open source toolkit implementing the Secure Sockets Layer (SSL v2/v3) and Transport Layer Security (TLS v1) protocols as well as a full-strength general purpose cryptography library. The project is managed by a worldwide community of volunteers that use the Internet to communicate, plan, and develop the OpenSSL toolkit and its related documentation.
TrueCrypt
TrueCrypt : Open-Source Disk Encryption Software for Windows and Linux TrueCrypt is an excellent open source disk encryption system. Users can encrypt entire filesystems, which