is an implemention of SSL and TLS protocols with full-strength cryptography world-wide.
OpenSSL is based ont the SSLeay library.

Not only an SSL/TLS implementation, but also a portable library full of all those cryptography functions you need. It handles all those SLA's, like RSA, DSA, X509, PKCS#*, SHA-1, MD5, PEM, S/MIME, etc.
The only con is: It's a nightmare to program to, the API is mostly undocumented and is rather complex.