Stunnel is a program that allows you to encrypt arbitrary TCP connections inside SSL (Secure Sockets Layer). Stunnel can allow you to secure non-SSL aware daemons and protocols (like POP, IMAP, LDAP, etc) by having Stunnel provide the encryption, requiring no changes to the daemons code.
The Stunnel source code is not a complete product - you still require a functioning SSL library such as OpenSSL or SSLeay in order to compile stunnel. This means that stunnel can support whatever (and only) that which your SSL library can, without making any changes in the Stunnel code.
