Gnu Pretty Good Privacy

PGP is a very powerful encryption tool and is open access, more on OpenPGP.  There is also the Gnu equivalent for command line based file encryption known as the GNU Privacy Guard (GnuPG). More information can be found here at their main website. GnuPG is available for Windows, Mac and BSD. Mac users may also want check out GPGTools.org for streamlined installation. A howto build for Mac installation is provided here.