Editor: If you are looking for a free Hylafax(tm) desktop client here it is.
You can fax trough your Hylafax server any document on you Mac OS X desktop, even just printing on the virtual printer or trough Drag & Dropping a pdf document on it.
The integration with the MAC OS X Adress Book will permit you to access directly the fax number stored in it or input a new fax number on the fly.
You will be able to setup the notify by email to have hylafax notify you when the fax documents are done, and even preappend a prefix to access the Telephone line if yours pbx require it.
This application is completly free.
From his interface is possibile to access the links to post feedbacks or request they are really wellcome.