What Mac are you using? Mac's that have built in modems have faxing capability built in. I've used my iBook to send faxes. The controls are found in the Print dialog box. I think you need a modem to fax stuff, so if you don't have a built in modem, I don't know what you can do other than buy an external modem.
Many places like kinkos or office depot offer faxing services, that would probably cheaper than buying a modem.