Cocoa eFax, from Ben Mackin, is a simple, elegant user interface on top of the UNIX eFax program.
eFax sends PDF files, which thankfully is supported so well by OS X that any application can save to a PDF via the Print dialog.
Simply launch Cocoa eFax, enter your phone number, add your PDF files and off it goes. I sent a fax and it went through cleanly, even after the other end picked up and said “hello? hello?” and then started the fax machine on their end.
I have not tried receiving a fax. There is also an untested “Scan and Fax” feature.
In the “Send Fax To:” field you can enter a name “Riggins” and click Check Address Book. The popup menu will then list matching items, however it shows them even if they don’t have Fax Numbers listed. Only choosing the name will report the error. Cocoa eFax should take a page out of Mail.app and use a nice palette for searching for a name (complete with x in the search field) and only match on records with Fax: numbers.
Cocoa eFax is shareware $10, quite affordable, requires no extensions to run and allows you to send 5 faxes before paying.