A quick Google search yields plenty of pay-for faxing services, but TPC Fax sends faxes from your computer over the internet for free.
Send a plain text fax directly from the TPC site or with your regular email client. For formatted documents, download the TPC client software which prints from any application to fax. The catch is TPC fax only sends to certain locations, so refer to the coverage list or just check to see if your destination number is covered first.
TPC Fax [via Ask Metafilter]
Download client software for remote printing [TPC Fax]