OpenSSL

OpenSSL is the open source library that has virtually been raised to the rank of a UNIX standard for its cryptographic functions. In particular, it completely implements the standard for PKIs, i.e. X509.

In addition to providing for encryption (and any possible hashing) using any known algorithm (MD5, AES, Blowfish, etc.), it can also issue and revoke certificates using X509 and is fully capable of handling the certification chain. It can write and rewrite certificates in the most common formats on the market, and many tools that are more advanced in terms of their usability are based on OpenSSL.
The development of OpenSSL is old, dating back to before 1998. It is available under either the Apache or the BSD License.
Developed in C, it remains the benchmark for Linux and BSD and is often used in all sorts of situations requiring encryption (ranging from 802.11 protocols to HTTPS communications, by way of SSH and FTPS).