Kindi is a lightweight command line-based tool that allows you to encrypt files that only the recipient you specify can decrypt (also using Kindi).
In order to do so, Kindi uses public key encryption. Unlike other public key infrastructures it doesn't need certificate authorities or certificate signing parties.
It's much simpler to use since it doesn't require users to manage certificates and keychains.
Authentication with Google Gmail should be good enough to prove somebody's identity, so Kindi generates self-signed certificates that are stored as PNG images in special public Picasaweb albums.
Detailed instructions on how to use the Kindi utility on your Mac are available HERE.