FAX90 Client is a free Hylafax(tm) desktop client.
With FAX90 Client 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.