efax is an integrated fax program for Unix-like computer systems, produced by Casas Communications Engineering since 1993.
[1] The software allows users to send and receive faxes using a computer, fax modem, and telephone line.
It is command-line based, but there are several graphical user interfaces available.
[2] efax is open-source and free software, licensed under the GPL.
[3] There is also a BSD version, and it forms part of the Apple Mac OS X Darwin system.