There are a number of fax programs out there that will let you fax and
receive documents. One of the most complex is Sam Leffler's
HylaFax, available from ftp.sgi.com. It supports all sorts
of things from multiple modems to broadcasting.
Also available, and a better choice for most Linux boxen, is
efax, a simple program which sends faxes. The getty program
mgetty can receive faxes (and even do voicemail on some modems!).