Tcpcrypt

Tcpcrypt is a protocol that attempts to encrypt (almost) all of your network traffic. Unlike other security mechanisms, Tcpcrypt works out of the box: it requires no configuration, no changes to applications, and your network connections will continue to work even if the remote end does not support Tcpcrypt, in which case connections will gracefully fall back to standard clear-text TCP.

Tcpcrypt supports Linux, Mac OS X, Windows, and FreeBSD.

For more information, see tcpcrypt.org.