Asterisk is an Open Source PBX (or PABX) integrating PSTN telephone lines and VOIP into a single solution, providing all of the functionality of a high end PBX; and best of all its free!
AsterFax builds on the services provided by Asterisk to provide a full fledged email based Fax Gateway.

What that means is that with AsterFax and Asterisk you can send faxes from your desktop using your standard email client without have to install any software on your desktop.

AsterFax can translate a normal email message into a fax message. You simply enter the destination phone number in the 'To' address, compose your email message and click send. Its that easy.

For more formal correspondence AsterFax also supports a growing number of file formats such as PDF, Tiff, Postscript, MS-Word and OpenOffice Writer. You can either attach the file to your email message or for applications such as MS-Word and OpenOffice you can fax directly from the applications by using the 'Send Email' option. The resulting email is sent as a fax message.