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!).
There is an experimental service offered that lets you send an email message containing something you'd like printed such that it will appear on a fax machine elsewhere. Nice formats like postscript are supported, so even though global coverage is spotty, this can still be a very useful service. For more information on printing via the remote printing service, see the FAQ.