efax is a small, free, command-line fax program developed under Linux. It compiles with very little tinkering under 10.1, and I have successfully sent and received faxes with it using the internal modem on my Powerbook G4. efax communicates with the modem using POSIX /dev interfaces, so I don't see why it shouldn't work with just about any modem that is supported by OS X.
For now, installing and using it is a bit (OK, very) command-line intensive, but it should be easy to wrap this up in an installer package and add a GUI interface (which I might do if I find the time - but I wouldn't be upset if someone beat me to it). Fax capability is essential for OS X, and FaxSTF X from Smith Micro is a complete disaster (I am one of the fools that sent them my money).
I have written a brief document explaining how I have used efax and ghostscript to send and receive fax documents in pdf format.
[Editor's note: In fact, a free Cocoa front-end to efax has just been released, called Cocoa eFax. It includes the efax UNIX program along with the Cocoa front end, but I have not tested the program so I can't vouch for it one way or the other. I also haven't tested FaxSTF, so I can't comment on that package, either.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020103163437474