SSLeay was a library for doing SSL, written in C by Eric A. Young. SSL is a protocol developed by Netscape to secure comunications between web browsers and web servers. It's also used for stuff like POPS/SPOP and IMAPS; see also stunnel. He later started working for RSA to implement an SSL library for them, and thus quit working on SSLeay.
Other people continued to work on SSLeay, and it eventually became OpenSSL. OpenSSL now also includes TLS, an update of SSLv3, which was designed by the IETF.
SSLeay (and thus OpenSSL) includes a very fast DES implementation written by Eric, as well as things like RSA, Diffie-Hellman, IDEA, MD5, and lots of high level stuff like ASN.1, PEM, and S/MIME.